3 days of healthy eating and exercise then i just exploded and ate an entire medium pizzavI can't lose weight?

i was doing good eating healthy and exercising for 3 days and tonight i came home and smelled yummy pizza my sister made and i got so hungry, i kept fighting myself not to eat it and was doing good for an hour, but then i just exploded and i ate her entire whole pizza. now i feel full and good. but now im sad and depressed again because i ate the pizza. I was wondering if this makes the last 3 days worth nothing? I really want to continue but am i back at square one? How do i stop my cravings?