Don't think you are alone in this, you are NOT! I did nearly the exact same thing last night. I ate high calorie foods for lunch/snack (which left me with 300-400 calories to stay within range) and then ate pizza for a late dinner. I too ate half of a pizza (small veggie), which doubled my calorie intake for the day. I've only exercise once this week and don't feel like doing any today. I am discouraged, yet I'm not ready to throw in the towel.
I have the other half of the pizza lurking in the fridge and I plan to eat it, then consume lower calorie foods so that I stay within range. Some days are a tiny bit easier than others...today will be better than yesterday for both of us.