Kyniska,
First I need to correct something you said. You characterized yourself as weak and pathetic. You were NOT weak and pathetic, you were in serious pain because of deep betrayal. You acted in the only way that your mind could let you in all of the situations that came after the betrayal. You are still saying derogatory things about how you behaved. Yes they are embarrassing, but they are normal and completely acceptable responses. For this reason, I would recommend not going to the party. Once you can forgive yourself for what you seem to think was “bad” behavior, you will be able to interact with this person again without shame or discomfort. You have a ways to go in healing, take as much time as you need. You don’t have to prove you are over him, or strong, or not “a crazy ex”. That’s not your job. Your job is to learn from this, make adjustments to not let it happen again, and move on to someone who values you. Good luck!