If you've ever fallen off the diet and exercise bandwagon for a quick binge, a week's vacation or (gulp!) a significantly longer period of time, the diet books are full of advice. Most of it says something like: "Don't beat yourself up over your mistakes, just get back on track!" And while I certainly recognize that guilt trips are the scenic route to nowhere, I disagree with the idea that there's no reason to dwell on a mistake.

It's no fun to pick apart the moment when we ate all the leftover birthday cake. It's discouraging to remind ourselves that we blew off the gym for a month while sheepishly paying the membership. And it's hard to confront the expiring cartons of cottage cheese in the refrigerator and remember the dietary reforms that we swore we'd make when they were fresh. But to simply pretend these things didn't happen and blithely "just get back on track" is to miss an opportunity to master your behavior once and for all.

Analyzed carefully and without shame, mistakes are some of our best teachers. Let's take the "vanishing birthday cake" incident, for example. If you've never done this, I'm fairly confident you've done something like it: You bake a cake for a friend or relative. The birthday ends and you're left with the cake. You stash it in the fridge, promising yourself that while you may have a little of it, your kids/colleagues/husband are going to eat the lion's share. But a funny thing happens on the way to the lion's den, and somehow you end up eating most, if not all, of the leftover cake.