The weight of food

I find it amazing how much weight we put on food.
A friend is struggling with the fact that she ate bread over the weekend. Out of all the good things she did, whether it be with food, with exercise, spending quality time with someone, the one thing she focus' on and remembers about the weekend is that she ate bread.
The weight of eating it causes more damage then what the actual piece of bread will do to her body.
Why do we do this, but more importantly how do we go about changing our beliefs or preceptions of food?

I'm at a cross roads, I am in the process of determining what I can go back to and what I may need to stay away from after the surgery. Do I restrict myself and deny ever having bread again, or sweets, or chocolate? Or do I teach myself to venture forth into the world of moderation? Is that even possible when you view the piece of food as the enemy?
To quote Dexter - how do you forgive your enemy? Is that even possible?

And why do we tend to focus on what we deem as bad - eating that piece of bread, rather then focusing on the balanced meal we ate at lunch or the long walk we had in the sunshine, or the great visit we had with a friend?
In the long run that piece of bread isn't going to mean anything to us, so why does it carry so much weight?

H