If confronted you say, "Look I'm sorry I acted like an asshole last time I was confronted about this and that I deliberately tried to provoke you all. I do not intend to engage in a similar level of immature behavior again and I'm hoping we can all move on and that you can do the same. I realize that my actions have not been perfect here. I shouldn't have dumped a partner of a year over text and I very much regret my choice in doing that. I did not cheat, because in my mind that text made it clear we were broken-up, that I had chosen to use drugs over our relationship, and that we were both free to date other people. That said, I don't appreciate being harassed and called a cheater and I'm going to ask that it stop following this conversation. The way I have acted is in some respects inappropriate, but so is confronting me right now. I don't wish to discuss this further, especially at a friend's birthday party as today is about celebrating them. I'm glad we've all moved on and are in a happier place, I wish her the best, etc. But I really think everyone involved needs to drop this out of respect for the person whose party this is." Then you walk away.

You brought this on yourself by acting like an asshat the previous times you were confronted and by creating the drama. If you think the drama will really be that unbearable and ruin your friend's night than you should probably chose not to attend given your role in all of this. If you insist on attending, diffuse, own your role, hold yourself accountable (in other words apologize for what you did do wrong and outline steps to ensure you won't engage in similar behavior again), and then politely request that they leave you alone. If you try to rile them up than any drama that occurs is something you've contributed to.