Trust yourself to know that eating that [insert "bad food" here] isn't bad, it's not going to mess you up, and that you can stick to one bite. You'll hit the spot, AND show yourself that you can do this and can see it through.
I dance around the house while doing chores or cooking or whatever. I don't count it towards my activity log, I just do it for fun. Try not to smile while you dance to Don't Stop Believing. Just try it. Bet you can't.