It was all going so well but temptation has finally taken hold. Sarah Vine talks diets, dinners and falling off the wagon

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I am a worm, a mollusc, a repulsive spineless dirt dwelling creature. I have no resolve, no gumption, no discipline, no moral fibre. Yes, you guessed it. I have fallen off the wagon.

I honestly cannot remember how it happened. One moment I was congratulating myself on having found a new way of cooking quinoa , the next I was face down in a half-litre tub of ice-cream. Mint choc chip, as it happens.

It had been a long and difficult week. There was a mad woman at a train station who rattled my cage in front of the children. There were not one but two political dinners which could not be endured without the gentle fug of alcohol. There was tea at a friend's house where she served the most delicious sugar-dusted scones. With clotted cream. And jam. There was a pudding, cooked lovingly, that involved chocolate mousse inside a meringue. There was a strawberry pavlova. There were some wafer thin mints.

Now I feel sick. Not just physically, but with guilt and self-loathing. I don’t dare weigh myself. All I can do is stare gloomily at the departing wagon, wondering desperately whether my chafing thighs will transport me fast enough to allow me to climb back on again.

What makes it worse is that this is exactly what always happens. I lose a bit of weight, feel happy and confident about myself, and take my eye off the ball. I think I can handle it. But I can’t. I’m no different from any addict: one taste of the drug, and I’m straight back in the gutter before you can say Charlie Sheen.

What makes it even worse is that I’m such a cliché. There I was, proselytising about the joys of a gluten-free, sugar-free lifestyle to all and sundry, all the time just days away from a catastrophic relapse. One mouthful was all it took for the iron grip of discipline to slip.

This week, then, I have one simple mission. Get back on track.