psshpffftpsths tlojdasfbn!!! It's one day... Don't let it get to you. Everyone falls off the wagon, it's a hard thing to be "good" for a lifetime. All we know is that it's done and over with, you can't change stuffing those chocolate chip cookies in your mouth... but you don't have to eat those cookies tomorrow. AND you can definately push yourself harder this week to burn off those cookies! You'll be fine... don't beat yourself up. We are only human! And if you expect to do this and do this right, expect to eat some bad things every once in awhile.
I felt crappy after the holidays from eating that junk and not working out as much and then I was like, SCREW IT! I'm still going to reach my goal, dammit-any-how. There's no sense in beating myself up over it now. Just need to work harder at it. And also, it made me stronger.

Same goes for you!