Does one mistake ruin it?

Mistakes only have the power we give them. How we view them is mirror to how we view ourselves. Thomas Edison failed over 10,000 times before he invented the light bulb. We always learn more from failure than we do success. So, pick yourself up, log your calories and get back in the game.

I think the number one thing I have learned this last year is that this weight loss thing is more mental than anything else. If you get yourself in the right mind set, the rest should fall into place. I think a lot of us have done the guilt and self loathing thing. It's not the one cookie, or the single meal or even a couple of days. It's that we allow ourselves to let that guilt take over and then give up. So you over ate, it's not a biggie. It's a biggie if you give up though. The one time calorie overage won't do it, the quitting will.

Thomas Edison failed over 10,000 times before he invented the light bulb.

Obligatory pedant: He didn't invent the light bulb, he improved it. See http://en.wikipedia.org/wiki/Thomas_Edison#Electric_light