I once heard a story about a couple in a restaurant who ate in total silence for over an hour. When coffee came, the husband whispered something to the wife, who hissed back: “It’s not the coffee, it’s the last 25 years.”

A slow crumbling like that would be pretty appalling. But when you’re given the surprise approach, the moment of impact feels brutally physical.

Someone stands across from you, looks directly into your eyes and tells you they are leaving you, they no longer love you, they have found someone else, you are not enough, and you think: “Oh, so this is the moment I am going to die. I can’t possibly get through this.”

As I lay on the floor of my own sitting room, watching my husband’s feet walking quickly towards the door, I knew that the end of my marriage, after less than a year, would bring unbearable sadness, awkward questions, terrible embarrassment. I even knew that, with the right coping skills, it might be OK in the end. But I also knew something else: at 29, unlike most adults, I had no coping skills.

Anxious even as a very small child, I had let my worries fester, take control, and dominate my life. Mental health problems had stunted my own growth, leaving me too scared to take on challenges. I quit things when they got hard. I turned down opportunities that would push me, or give me independence. I preferred being small.

From a young age, I had been agoraphobic, prone to panic attacks, intrusive thoughts, hysteria and depression. By the time my husband walked out on me, I’d had years of this. Often I couldn’t make it to the supermarket on my own (honestly), much less navigate my way through a breakup of this magnitude. I knew I had to get off the floor, but I didn’t know what to do next. Everything was draped in fear.

If ever there is a trigger to make you try to change something, it’s the shock of your marriage collapsing. Given that people who get divorced in the UK have usually managed about 11 and a half years before they pull the plug, tanking your vows as spectacularly as I did felt like quite the feat.

Any longer and it might just have been seen as sad, unavoidable, or chalked up to “young people not sticking at anything anymore”; but eight months? It would be unwise not to question your life just a little bit after that.

I went back to work, alternately crying in the toilets (my husband worked for the same company; that was fun) and sitting mute at my desk, listening to bagpipe music on my headphones in a strange attempt to find some mettle whenever I saw him walk by. (As an aside, this was strangely effective and I would recommend it to anyone needing to feel strong. Start with Highland Laddie.)

I felt stagnant, aware that I had to endure these painful emotions, but also worried I might never feel truly better. Life continues around you, no matter how much your own world has been shattered. I could see normality heave into view and I didn’t want it. I suspected that, within a few months, I might be over the breakup but still locked in my small space, anxiety and depression my only bedfellows.

It’s easy to behave as if nothing is wrong, even when you have a mental illness. I was good at holding down my job, cracking jokes, going out just enough that I wasn’t seen as a hermit. I could probably have gone on like this for ever, living half a life, pretending I was OK with it. But something had broken, and I couldn’t do it any more.

I saw myself exposed as a fraud – a cowardly kid play-acting as an adult, with no business being there. JK Rowling has said that when her own short-lived marriage imploded, leaving her an unemployed single parent, rock bottom became the foundation upon which she built her life: because her worst fears had been realised, she had nowhere to go but up.

As it’s her, I can allow the cliche and even grudgingly admit it fits. In Rowling’s case, she went on to create a magical world of wizards that helped her become one of the richest women in the world. In mine, rock bottom spurred me on to go for a jog.

I still don’t know why running was the tool I opted for in the midst of misery. I’d never done strenuous exercise before. But I had spent a lifetime holding at bay the need to run away – from my mind, from my negative thoughts; from the worries that built up and calcified, layer upon layer, until they were too strong to chip away at.

Maybe the sudden urge to run was a physical manifestation of this desire to escape my own brain. I guess I just wanted to do it for real.

I was about to turn 30, and terrified I would use the breakup as an excuse to retreat, to be scared of life itself. I was not ready to run across a playing field. So I put on some old leggings and a T-shirt and walked to a dark alleyway 30 seconds from my flat. It fitted two important criteria: near enough to the safety of home, and quiet enough that nobody would laugh at me. I felt absurd and slightly ashamed – as if I was doing something perverse that shouldn’t be seen.

With my headphones in, I settled on a song called She F***ing Hates Me by a band called Puddle Of Mudd. Not to my usual taste, but the lyrics were suitably angry and I didn’t want anything that might make me cry (everything was making me cry). I managed 30 seconds of jogging before I had to stop, calves screaming and lungs burning.

I rested for a minute, and then started again. I somehow managed to keep time with the shouting singer, mouthing the words as I screwed up my face and lumbered down the path. I ran an incredible three minutes, in stages, before I gave up and went home. Did I feel better? No. Did I enjoy it? Also, no, but I hadn’t cried for at least 15 minutes and that was good enough for me.

To my surprise, I didn’t leave it there. I went back to that same alley the next day. And the day after that. Those first few attempts were all pathetic, really. A few seconds, shuffle, stop. Wait. Go again. Freeze if a person emerged from the shadows. Feel ridiculous. Carry on anyway. Always in the dark, always in secret, as if I was somehow transgressing.

I got shin splints, which hurt like hell. I ran too fast and had to stop after wheezing uncontrollably. I tried to go up a hill and had to admit defeat and get on a bus; I had a panic attack in a dark part of the local park when I mistimed sunset and realised I was all alone. I fell over and cried like a child.

Running felt like a language I couldn’t speak, and not only because I was hugely unfit. It seemed to be something only happy, healthy, bouncy people did – not neurotic smokers who were scared of everything.

Throughout my life, if I couldn’t do something well on the first attempt, I was prone to quit. It was embarrassingly clear to me that I was not running well, or getting better at it. And yet, much to my own quiet disbelief, I carried on. For the first couple of months, I stuck to the roads closest to my flat, looping around quiet streets. I was slow, sad and angry. But two things were becoming clear.

The first was that when I ran I didn’t feel quite so sad. My mind would quieten down; some part of my brain seemed to switch off, or at least cede control for a few minutes. I wouldn’t think about my marriage, or my part in its failure. I wouldn’t wonder if my husband was happy, or out on a great date, or just not thinking about me at all. The relief this gave me was immense.

The second thing, which was even more valuable, was that I noticed I wasn’t feeling so anxious. Soon enough, I was reaching parts of the city I hadn’t been able to visit in years, especially alone. Within a month I was able to run through the markets of Camden without feeling I would faint or break down.

When your brain has denied you the chance to take the mundane excursions most people do every day, being able to pass through stalls selling “Nobody knows I’m a lesbian” T-shirts suddenly feels like a red-letter day. By concentrating on the rhythm of my feet striking the pavement, I wasn’t obsessing over my breathing, or the crowds, or how far I was from home. It was miraculous to me.

When you run, your body takes your brain along for the ride. Your mind is no longer in the driving seat. You’re concentrating on the burn in your legs, the swing of your arms. You notice your heartbeat, the sweat dripping into your ears, the way your torso twists as you stride. Once you’re in a rhythm, you start to notice obstacles in your way, or people to avoid. You see details on buildings you’d never noticed before.

You anticipate the weather ahead of you. Your brain has a role in all of this, but not the role it is used to. My mind, accustomed to frightening me with endless “what if” thoughts, or happy to torment me with repeated flashbacks to my worst experiences, simply could not compete with the need to concentrate while moving fast. I’d tricked it, or exhausted it, or just given it something new to deal with.

Anxiety has been with me for as long as I can remember, but it’s ebbed and flowed over the years. At 11, I went to secondary school and the change sent me into a tailspin. I cried every day, like many other kids who hate moving to a new place and making new friends; but I didn’t stop there.

I developed OCD tics – swallowing whenever I had a bad or negative thought, blinking, even more disgustingly, spitting – as if to rid bad feelings from my body as quickly as possible. I had no idea what this meant – I just knew I “had” to do them. I remember missing my bus stop in the mornings many times because I hadn’t blinked in the correct way. There was no winning; the goalposts would shift all the time. If it wasn’t blinking, it was avoiding cracks in the pavement – small things that paralysed me.

These routines would take up hours of my time, partly in the doing and partly in the concealing; those around me must not know. I also found myself disassociating for the first time – detaching from my surroundings when it all got too much.

This remains my most terrifying anxiety symptom, and the one I can’t totally shake; though it’s believed that your brain does this in an attempt to protect you, it only makes me feel much worse, as though I’m drowning but my legs don’t work. Colour gets too bright, sounds are jarring and it feels like I’m cocooned in bubblewrap, unable to get back to reality.

At worst, I’ve looked in the mirror at my own face and not recognised it to be me, and not just because I had terrible hair and bad skin that morning. It’s a strange and awful experience. When I was trapped in a fug of anxiety and depression in my early 20s, disassociation made it feel as though the people around me were actors in a bad reality show. I couldn’t connect with loved ones; everything felt fake and staged.

What else? Well, I would scratch and pick at my skin, until it bled and scarred, pull out hairs (a mild form of trichotillomania, where sufferers have an intense urge to pull their hair out and feel a strong sense of relief when they do). I’d chew my lips until they bled. All fun scars to have as an adult: “Why do you have scars all up your legs, Bella?” “Oh just because I pull and pluck my leg hair until I bleed when I feel like I’m losing control – who wants another drink?”

Having managed to leave school with most of my childish worries fairly dormant, I was knocked off my feet one day at university, when, out of the blue, I had a terrible panic attack. The clever (not a compliment) thing about anxiety is that the moment you’ve got a handle on one thing (night sweats, panic attacks, dizziness, nausea, headaches), it’ll throw you another one, and you better believe it’ll be worse.

Running is not a cure-all for severe mental illness, or anything else for that matter. But I often think of the girl I was in my 20s and wish I could go back and try putting on some trainers. Instead, I dropped out of uni, went to a psychiatrist and took the antidepressants that I was swiftly prescribed. What else could I do? At this point, suicidal thoughts were creeping in.

Despite all of this, I was extremely fortunate. I had a family who, while not fully understanding why their daughter was crying hysterically all the time and refusing to go out, had the resources to pay for me to see a professional. (My NHS GP was kind, but could only put me on the waiting list for therapy.)

The pills helped, and I was able to look at myself in a mirror again without wondering who was looking back at me. After quitting my degree, I got a job, was able to go out again, and managed a few relationships. I was patched up, in the most basic sense.

I say all this, not to give you a small insight into my not-particularly-special mind, but to show how easy it is to accept the most pallid imitations of existence when you’ve got a mental illness. To paint on a small canvas, and to pretend that you’re happy with the narrow perimeters you’re able to move within. Not a life wasted by any means, but a life limited. So to find something that breaks you free of this can feel miraculous.

For some that may mean medication, for others meditation. My mother does yoga whenever she feels low. A colleague lifts weights, and one friend boxes because he feels far too angry and it helps keep those thoughts under control. Somehow, in the wreckage of my marriage, after a decade of settling for merely “managing”, I’d found the thing that broke me out of it: I’d found running.

Weeks after my marriage collapsed, I was still sick with it all. At work, I would regularly go into the toilets and cry quietly. At home, I would put on my pyjamas the moment I got in and mindlessly watch TV. When I went out, I drank too much and would cry again. While I was running, nobody could give me the dreaded sympathy head tilt or an excruciating hug. Nobody even looked at me.

I soon found I was setting myself little challenges: go two minutes farther today, run down that busy road you’ve avoided for years. I discovered old railway lines that ran like arteries through built-up estates, hidden from plain sight. I ran along the canal and found an expanse of brambles, wild flowers and ducklings swimming along next to me. The panic attacks were fading away.

One day, I decided to go farther. I ran into the heart of the city, towards one of the bridges that traverse the Thames and beckon you over with the promise of light and air, and I headed across without a backwards glance. I crossed another bridge, intoxicated by the sunshine on my skin, and I ran into Parliament Square, thronging with tourists and vendors and honking cars. I passed through Soho, marvelling at the noise and rickshaws and sex shops.

I kept going, like a neurotic Forrest Gump, until I physically couldn’t go any farther. And when I stopped, I wandered around. The pit in my stomach wasn’t raw, I wasn’t checking my breathing – I didn’t notice my body. I was able to take in my surroundings and enjoy them. I felt triumphant. I felt… happy.

Running is not magic beans. Life is tricky and gets diverted constantly, and we all stumble. There have been crappy times. There have been brilliant times. But the main difference between my life before I ran and my life since is that I have hope. And I have a life that is not always dictated by worry, panic, doom and depression. You can do so much more when those things don’t sit on your chest and slowly squash you.

Some people might take my (small) achievements as proof that I simply grew out of my anxiety, or that I was never affected by it too much in the first place. I assure you neither is true. Anxiety rarely “leaves” you. Some people might be lucky and feel it float away one day; but for most of us it’s a lifelong companion we must learn to live with. That doesn’t mean enduring it, or giving in to it. It means finding ways to negate it, to push it back.

Since that first short and sad run I took over four years ago, I have lived alone, travelled, changed jobs and begun a new relationship. Knowing I could do a 10K meant I knew I could fly to New York for a job interview, and that I could step outside my door alone without hyperventilating.

It’s a measure of how over the whole “starter marriage” I am that I sat across from my boyfriend at dinner last year and proposed to him (he said yes, thank the lord). Running has given me a new identity, one that no longer sees danger and fear first. I ran myself out of misery.

Six tips for anxious runners

Take water Most experts say you don’t need to, on short runs, but it might help if you get panicky and need to stop. Take sips, wait for your breathing to get back to normal. I have a bottle that moulds to my hand and makes me feel I’m carrying a neon weapon.

Podcasts and music help They distract me when I get bored, or tired. More importantly, at the beginning, they made my brain concentrate on something other than worry.

Start small If leaving your safe places makes you feel vulnerable, do a loop of your road. Run that road until you feel confident you can go to the next one. It all counts, and it’s important you don’t push yourself too fast. Listen to your body.

Nobody is looking at you Running feels incredibly exposing, overwhelming and scary to begin with. I assumed people would mock me, honk from vans. But nobody batted an eyelid. I fell over at the feet of a man on the canal path and he carried on eating his sandwich.

Enjoy the beauty around you Your anxiety can make you introverted, forcing your brain to see negative, scary things instead of your surroundings. Nearly every time I go for a run, I stop to take a longer look at a building, a poster, a sunset. My phone is full of photos of weird street names, beautiful views, and dogs I see along the way.

Be kind to yourself Buy an ice-cream after a run; have a glass of wine. Never berate yourself if you have a panic attack and need to go home abruptly. Running is not always a straight line (that would be boring).