So, I've been under a lot of stress lately. I also started my TOM yesterday and I've been craving pizza like you wouldn't imagine...thinking about going off plan for one pizza meal which would probably throw me out of ketosis (I'm on a low carb plan) and screw things up. My losses have been incredible at 4lbs this week and I've been eating lots of vegetables and feeling great about my plan.
Tonight my family had pizza. I'm this close to having an emotional breakdown, but mostly I was just hungry. I had turkey burgers planned but didn't feel like cooking them so I decided to have the topping off of one slice of pizza. Not exactly a nutritious meal, but also not exactly off-plan because all of the foods are allowed (sausage, cheese, pepperoni). The only thing I was worried about was the sauce which may contain sugar. A bit later I went and got in the shower.
When I got out of the shower I said something rude to my grandma because I'm not in a good mood and I really upset her which I feel bad about. My mom has a migraine and is brushing off things that I need to ask her, and when I went into the kitchen the pizza was staring at me. I ripped the toppings off of another slice, and then starting picking the pieces of pepperoni off of a bunch of them. I ended up eating the toppings off of a third slice. I thought briefly about going and throwing it up...which I know is a horrible thing to consider but I just felt so guilty for eating it and now I'm afraid the scales might be up tomorrow because of it.
Right now I'm having a small salad with ranch because I felt like I needed to get some more veggies in today after eating all of that cheese. I was doing so well with almost completely eliminating cheese, too.


Don't let it happen again, and you'll be great! Today is just one day -- you've done 1/2 a pound of damage max -- and you will (you WILL) work out an extra half-hour tomorrow to start taking off the damage. 
The more we deny...the more stress we induce...stress raises cortisol...cortisol aids in fat storage!
Additionally, allowing yourself an "off-plan/cheat meal" once a week/bi-weekly increases your chances of LONG-TERM success! It also helps to keep the body guessing so as your weight reduces...you don't have to keep reducing your calories!
(3 slices = 525 cals.)