OP,
I am so sorry you are feeling so low.
I think there are a few things going on.
First off your partner is not a good man.
He's weak and spineless and he certainly doesn't deeply care for you.
If he did he wouldn't want to be near your Ex and he certainly wouldn't stand by and have you treated like that.
Good men don't behave like that.
Weak selfish ones who put themselves first do.
I absolutely do not think you should be going.
I think your partner is protecting himself and you are too.
Who has YOUR back?
He certainly doesn't.
Your ex did something terrible to you and your partner is content to stand around chatting and watch you humiliate by a piece of shit.
That isn't a partner.
He has no loyalty to you.
Don't invest in someone who clearly doesn't make you his priority.
As for those friends🙄 that have complicated things?
Step away.
In your place I would tell your partner the truth about the night and the way things have turned out.
Tell him that you have absolutely no wish to be in your Ex's presence after his treatment of you.
Let HIM make his decision as to where he wants to be.
If he chooses to be with this person who treated you so poorly, I think you have your answer.
I really am sorry life is so hard, but investing in a relationship with a weak spineless man is never satisfying long term.
Loyalty, respect and integrity are the cornerstones of a good relationship.
He doesn't have those qualities.
He may be nice and fun etc., but when it comes to it, he is weak and will suit himself completely, whatever makes HIS life easier.
You deserve better than this in a man and partner.
This issue will not go away.
This is who he is.
You deserve better than him.
Don't put yourself through this for him, he really isn't worth it.