I am sorry you feel lost. I know it is hard to feel.
the whole point of multiple relationships is that an existing relationship does not have to be shunted aside/abandoned for a new one, yet that feels like exactly what he's done.
Yep. I don't have
to punch you in the head. But if I go ahead and do it anyway? Well, I did it, didn't I?
Coming to terms with the fact that he is going to do as he pleases with no consideration for you is hard to come to terms with. He says one thing (I love you) and then does another (less than loving and kind behaviors).
You could accept that is what it IS here. Mixed message land and when that happens -- you go with ACTIONS because talk is cheap.
You don't have to love it that it is happening. It just IS happening.
If you don't want to go to this social function because things between you are not cool, do not go at all, or do not go as his date. Go alone. Take someone else.
Protect yourself, ok? He's not going to. It is up to you then.
Swim for shore. Don't get new punches in the head.
Hang in there!