I'll be honest: it started badly. There was a small waterfall upstream and then a long tail of fast-flowing water with a few flutters of whitewater on it, then broad calm pools on either side. We were in one of these, sitting in kayaks no longer than a tall man, all sealed up with dry suits and crash helmets and spray decks. I'd heard the instructions for what is called a ferry glide: lift your right knee so the kayak digs into the downstream water, look where you want to go – the pool on the other side of the flow – but keep the boat pointing at about 45 degrees upstream. Paddle strongly. I'd seen it being done, slowly and expertly by Jasmine. "What we are learning is to be in control in whitewater."

It took only two powerful strokes to hit the edge of the fast water. The kayak gave a wobble. The bow swung swiftly and unstoppably downstream. My knee dropped. My hands lifted. The wobble became a roll. The freezing water of the River Llugwy came up to meet me. In that fraction of a second between incompetence and impact, there was time for several thoughts to flow through my head, faster than the river was moving: "Oh no, this will be cold!"; "How embarrassing!"; and, I'm ashamed to admit, "Will I be allowed to sit in the car with a warm drink for the rest of the day?"

I have always wanted to learn whitewater kayaking. Watching the Olympic hopefuls negotiate boiling rapids, apparently without effort, has an irresistible allure. It's that sense of a person totally alive to the water, using its currents and energies and yet totally in control, turning maelstrom into meditation.

Upside down in a Welsh river in winter, I mused on the gap between ambition and actuality. The icy fingers of water slipped around my neck and down my back. I pushed hard with my feet but remained in the kayak. There was a strong sensation of being swept downstream, of the clock ticking before the next set of rocks arrived. I pushed harder. The spray deck parted from the kayak, the water grabbed me and I twisted out and into the air. Rather unexpectedly, I was not 200 metres downstream on the lip of a giant waterfall, but in the pool on the far side of the river, the very place I had been hoping to go.

Jasmine was there too, pointing her kayak at me. "Just grab hold." We went to the bank and had a quick debrief on where I went wrong. She made it sound like a minor glitch. I got back in. No retiring to the car with a coffee then.

This time I took Jasmine's advice and kept paddling. The kayak wobbled horribly. It was not elegant. Brute force was applied. I arrived in the calm pool. I wish I could say this success bred confidence, but it did not. What I was thinking was: maybe this is the moment when you discover an activity at which you are totally, inexplicably, and unbearably crap.

I have always had the attitude that a fit and healthy person can do almost anything, given the right tuition, equipment and the requisite 10,000 hours of practice. But sitting in that wet, cold river, it was not only my kayak that was wobbling. My belief in this attitude was shaking too. Some things are simply too difficult, I told myself. A lifelong inability to balance is against you. A dodgy knee is against you. A bad back and a dud eye. A lack of moral fibre. And it's cold.

Jasmine appeared to know what was going through my mind. "Good," she said enthusiastically, as though I had just completed the Olympic slalom course in record time. "Let's quickly swap you into a different kayak. It might suit your weight and height better." She didn't fool me. I was being demoted to the barge.

Back in the quiet eddy pool, we resumed. "Do it again," said Jasmine, "and remember to look where you want to go. And when you wobble, don't stop paddling."

I was starting to notice certain things about this water that had escaped me at first. It was not just a broad road of fast water sliding through two slow pools, there were subtleties to it. On one side the borderline between waters was sharp and distinct; on the other less so. Getting across that sharp border was more difficult. There were submerged rocks too, although only in the side pool, and each caused a flurry of turbulence. I had to concentrate on my balance: lean forward slightly, keep an eye on my knees, keep kayak facing not where my face was pointing. Hang on, which knee should I lift? Was it right? Or was it left?

With an audible sigh of exasperation, the kayak barrel-rolled and I was upside down in the Llugwy once again. This time the water did not feel cold. I was used to it. Perhaps, I pondered, as I was swept downstream, I could just stay here, upside down, until I'm shipwrecked, and then creep away into the reedbeds and die. But the river didn't want me either. It hauled me from the boat and spat me up to the surface. Jasmine was there again. "I've never seen anyone manage to roll that kayak," she said, with a hint of admiration, then added, "Let's try it a little further downstream."

For the next hour I worked at it. Slowly I improved, then progressed to the faster water again. Now the wobbles were not always present and I'd learned the trick of powering through them. Jasmine taught me how to "break in" and "break out", essentially the art of paddling into fast water, following it downstream, then cutting out of it. Occasionally, just for a few seconds, I had the impression that I was in control and not purely due to brute force, but to guile. And, although even entertaining this observation convinced me that I would capsize immediately, I did not.

Next morning we were back. There were stinging sheets of rain coming downriver and some nasty gusts of wind, but we set out to kayak for a few miles. "The idea is that we always look at what's ahead downriver, spot any obstacles or dangers, and any useful eddies that we can aim for."

There were plenty of the former: overhanging trees, great rocks with piles of water creaming up their faces, islands and hidden twists. The Llugwy is a beautiful river, famed for its Swallow Falls (which can be kayaked, incidentally, if you are sufficiently deranged). It also happens to be the river with the highest rainfall catchment in England and Wales. Phenomenal quantities of water can power down the flanks of Snowdon and into its twisting rocky course, eventually joining the River Conwy. It can suddenly rise or fall, altering the dangers at any moment and the degree of difficulty. Unlike a manmade course, alertness to nature is required.

We angled our boats carefully to dodge a fallen tree, then swung into a small eddy by a fast race of water. This was a gorgeous spot and I was actually enjoying myself. Again and again I worked my way across the race. On what must have been about the 20th crossing, Jasmine nodded and shouted: "That was excellent!"

I sat up. I had crossed without thinking. Nothing. There had been no wobbles and very little effort. Somehow I'd managed to slip across the rapids without engaging any serious mental or muscle power. "That's it!" said Jasmine. "That's why we do it! That feeling."

It was a sweet one. It didn't recur, not quite. And I had a wobble on the next section of river. But when we pulled the kayaks out at the end of the day, I'd regained my confidence in the human ability to conquer new challenges. With the right tuition and equipment, I had done it. All I need is a further 9,990 hours of practice and I'll be ready.

• Jasmine Waters from Activ-8 Adventure (0845 4969177, activ-8.org/wwater) provided Kevin's whitewater kayak course; she runs two-day courses in Snowdonia from £240 including accommodation, meals and equipment. For further information on kayaking, see the British Canoe Union (bcu.org.uk)