First thing to remember is not to give up! Everyone screws up every now and then! Just because you make one mistake doesn't mean you failed in some way or you need to start all the way at the beginning. Just pick up where you left off again and you'll be on your way.
What diet plan are you doing? IMHO if you're being too restrictive on yourself right from the beginning it might be hard to stick to it long term, but that's just my opinion. It's easier to make a list of the things you want to change, then change them one or a few at a time until they become habits.