Guess what? I ate white starches and French fries last night. Am I fat? No. Did I lose all the progress I made? No.
Having one unhealthy meal to treat yourself isn’t going to do anything bad to you. If anything, it helps you to stay on track and remember why you live the way you do. So don’t punish yourself if you have one unhealthy thing once in a while – you won’t get in shape from one salad, and you won’t get fat from one burger or poutine.