Hugs honey. Having the pizza will only make things worse as you said yourself. What you need to do is draw a line under it all, pick yourself up and start again, don't be ashamed, we're all human and make mistakes and will only lead you down the path of pizza and a downward spiral.
Sit down and write yourself a list of what is stopping you sticking to plan and think about ways of over-coming them that you can then put into action when you find yourself slipping.
We all have lives to live where you want to just enjoy the moment and not worry about losing weight, the thing is to let them be an occasion rather than a regular occurance and then when over, get immediatly back on the wagon - you may have a gain if taken to too much excess, no guilt, just looking at the current and future healthy choices.
I know exactly how you feel hun. I've tried for the last goodness knows how many years to lose weight. I do well for the first couple of WI and then I think I might have put on so I don't go to meeting and then I give up.:sigh:
Try and go to wi on Monday, when i have had a bad week i am aprehensive about going to wi, but when i get there i know that i have faced it and can move on. I come away from group feeling focused and positive, if i avoided going i am sure i would do more damage and probably give up.