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.