Many times people will snack on unhealthy foods during the day. Most of the time when you are snacking, you are really thirsty, and eating will just put extra calories into your body. Try to drink a large glass of water when you are hungry, and you may find that you no longer have the urge to eat.
Dieting means you're no longer required to be a member of the "clean plate club". Don't be afraid to throw a few bites away. It can mean a lower calorie count in your belly and less fat on your waistline. If you absolutely can't throw that food away, share it or pack it up.