A month into my new life and Sam, my personal trainer, has left me. I am heartbroken. What have I said, or done, or not done, eaten or not eaten? I have been doing my flat-stomach exercises, sit-ups morning and night, but abs sense does not appear to have made the heart grow fonder. How could he do this to me? And just because his girlfriend is visiting from Chicago for a week. Is he in this for keeps or not?

The appeal of the trainer is threefold. You go to the gym to take control of your life: to exercise properly, eat properly, drink properly, get your mind and body into some sort of harmony. You go briefly to escape: the music is that techno-ambient stuff they play in nightclubs, all spacy and trance-like - exercise as drug. Movers and shakers come (a) because they can afford it; but (b) because it is a place where they don't have to do any moving and shaking - instead they are moved and shaken. (Here we have a paradox: they are taking control of their lives by ceding control to a trainer for three hours a week; prominent in life, they choose to become anonymous in this techno-industrial environment.)

But you also go to be loved. I haven't had so much attention since I was about five, when I had to leave my mother clinging and weeping (me, not my mother) to go to school. For one hour, three times a week, Sam talks to me, asks me how I am, tells me how to exercise, cajoles me, laughs at my jokes, flatters my ego, scribbles in a policeman's notebook, cares. At least I thought he did.

In his absence, Lucy has been looking after me. Lucy is a former international swimmer and has been putting me through my paces in the pool. For about 30 years, I have been labouring under the delusion that I was rather good at breaststroke. Once, in Greece, I swam out too far and found the current against me. A Shelleyan death beckoned, but my trusty breaststroke got me home. Now I discover I can't do it after all: the stroke is too fast, the legs insufficiently extended, the breathing all wrong, and my head never goes in the water, which strains the neck. It has to change, Lucy says.

But start to think about it - about how to breathe and pull your arms back at the same time - and the whole thing disintegrates. I am currently at the disintegration stage and will not be visiting Greece this summer. Lucy has explained the theory but the practice, especially the breathing, is harder. I keep inhaling under water with disastrous results: I used to be able to do about 30 lengths (very slowly, frequent stops); I can now do about half a length before spluttering to a stop.

But the darkest hour, etc, etc. I know it will be worth it in the end. And I look great in the new goggles, don't you think? (Film critic Peter Bradshaw, photographed last year at his local lido, pioneered this trend in Guardian writers appearing in swimming apparel, and I think it could be a real circulation-booster. There are apparently moves to extend it soon to the comment pages.)

Happily, my front crawl is faring better: just adding goggles, slowing down the stroke and not waving my head about wildly has improved it by about 30%.

Swimming is extremely hard work - halfway through these one-hour sessions I was really tired - but is reckoned to be a superb way of exercising: all the muscle groups get used and it is good cardio-vascular exercise too.

Here are a few tips, courtesy of Lucy:

• Unlearning bad habits is tough, so it's worth teaching children to swim properly.

• Try to swim for half an hour two or three times a week.

• Goggles - to help you swim in the water, rather than on top.

• Breathing properly is the key to swimming well - practise your breathing at the side of the pool, dipping your head in and out of the water to establish a pattern.

• Running in the pool - with your feet off the ground - is a good form of non-impactive exercise.

• Using a float and going up and down the pool by just kicking your legs will strengthen leg muscles. (It's tiring, though: I had a long walk one afternoon after one of these sessions and found it very hard going, my legs almost seizing up.)

Lucy is tough on fat - and tough on the causes of fat. Having discovered that one Saturday I ate sausages for lunch and a pizza for dinner, she gave me a daily food and drink chart to fill in, asking me what I ate, whether I was hungry, and whether I enjoyed it. Filling in the form was such a chore that I was inclined to eat nothing, but that approach is a disaster since your blood sugar level falls and you then stuff yourself in a bid for instant energy.

Little and often is the preferred routine: a hearty, healthy breakfast (muesli rather than bacon and black pudding, naturally); mid-morning fruit; a jacket potato and salad lunch; mid-afternoon fruit (take those giant-sized Mars bars away); fish or chicken in the evening, with fruit salad and yoghurt (preferably one of those enzyme-filled biological ones).

My food sheets of course looked nothing like this. Lucy said I ate too much too late (she suggests eating little or nothing after 8pm); I had had a box of popcorn at the cinema (nutritionally not too bad except for the high salt levels); and had stuffed nuts and olives at a party (nuts are only good if you eat them straight from the shells). Lucy said I should give up coffee completely, cut out sugar in tea, eat less bread - it's more fattening than you think - and spread the butter very thinly.

Look, thanks Lucy, but perhaps two-coffees-max Sam wasn't so bad after all.

• Stephen Moss is spending three months working out at Matt Roberts At One, 1 Aldwych, London WC2. Tel: 0171-300 0600.