It was last September, while on a hen weekend in north Wales, that I realised how important swimming had become to me.

There was a temporary break in the organised fun and a clutch of attendees I vaguely knew from university asked if I would join them for a dip at the nearby beach. The conditions were hardly dreamy: it was raining; I hadn’t bought a bathing suit; the only towel I could use was the child-sized one provided by the Airbnb. But I took them up on it all the same. “I’ll be with you in five minutes,” I said, grinning.

In the past, I would have required more coaxing; spontaneous, sociable swims were hardly my default mode of exercise. I’d suffered from anorexia in sixth form, and although I’d recovered while at university, in my mid 20s I’d started to slip back into that old, punitive mindset. Supposedly “healthy” eating plans were accompanied by a strict exercise regime: running. Five times a week. Solo. (Just like when I was a calorie-counting A-levels student, I craved control. Having someone join me would require compromise and flexibility – two concepts that seemed about as palatable to my weight-conscious self as supersize tubs of lard.)

I felt the relief of letting forces beyond yourself – the weather, the seasons, other people – dictate your agenda

To clarify, this wasn’t a constant period of anguish. Rather, it was three years or so in which I’d flit between nervy months of restraint and periods of respite when I somehow felt grounded enough to … well, chill out a bit. In these stretches, I could skive my runs entirely – or ditch them for a more leisurely form of exercise. I started going for lazy, pleasure-driven swims, and in doing so clutched at a lifeline that would eventually haul me from the whirlpool of self-criticism and obsessiveness for good. (Well, almost.)

As a child I had enjoyed visiting my local pool, but seeing as you can hardly open a newspaper nowadays without someone gushing about wild swimming, the adult me gravitated outdoors. On trips to visit family in Sweden, I floated languorously around calm, cavernous lakes. And like all good arty millennials in London, on boiling Saturdays I made for the ponds on Hampstead Heath.

It wasn’t until the hen party, though, that I realised how profoundly swimming had changed me.

Tragically, a women-only weekend like that – one involving glamorous outfits and carefully apportioned “getting ready” time – would normally bring out the worst of my neuroses. But instead of glaring at my thighs in the mirror, I was bobbing up and down in the sea in greying underwear, too busy squealing at the cold to think about anything else.

The physical benefits of wild swimming are well documented. But for me it was about more than the rush of adrenaline that comes with charging into a freezing sea, or the blissed-out peace you feel while towel-drying your hair in the cool evening air. Swimming outdoors, it turned out, had fundamentally recalibrated my mindset around exercise. After all, unlike running, it wasn’t an activity that I could control and schedule. Unless you’re phenomenally hardy, it’s weather-dependent – and, equally, unless you happen to have a lido on your doorstep, it hinges on location. You do it with friends, and having company blasts any perfectionist routines (“If I don’t do blah lengths in blah amount of time, I will have failed!”) out of the water. I started to appreciate the relief that comes with letting forces beyond yourself – the weather, the seasons, other people – dictate your agenda. While it’s no bad thing to take your fate into your own hands, I had a habit of squashing it once it was there. Swimming taught me the good that can come from letting go.

It also helped me to embrace my body. After all, in the water, plumpness is power. When I was at my lightest, in school, I was cold all the time – and swimming, particularly outdoors, was out of the question. But now I can do lengths and lengths without a worry. I’ve stopped dieting, and while numerous factors brought that about, the way swimming made me feel more comfortable in my own skin undoubtedly contributed.

Nowadays, I get into the water as much as I can. But it isn’t a strict routine; I approach it as a treat, like going for an ice-cream. While I still run, swimming has helped me see the benefits of exercising less intensively. I do a slow jog a couple of times a week – and most of the time it is motivated by a desire to let off steam rather than a compulsion to shed calories.

I’d be lying if I said I had shaken off my anxieties completely. I suspect I will always have an obsessive side. Happily, though, swimming is not only helping to keep this under control – it is also the result of doing so. If I’m relaxed enough to jump into the waves on an overcast Saturday, chances are, life is pretty sweet.