You might need to change your mindset a bit.
Is it that you ate the pizza, and see it as "evil", or that you went over your calorie range for the day?
If you went over your calories for 3 day in a row, then yes..you need to nip it in the bud. One day? Meh. Sh*t happens. Are you perfect? If not, don't sweat it.
You've realized that the mistake was not eating...then grabbing whatever because you were so hungry. Now that you know how to avoid it in the future, you take that knowledge and move on. Today is a new day.
You won't be able to change habits overnight and there will be slips along the way. It's normal and natural, and nothing to beat yourself up over.
The best exercise in the world is to bend down and help someone up.
| current weight: 111.0