Opinions on this please

What is going to happen at this wedding that is so painful for you?

Yes, you're overreacting. His friend's wedding isn't about you and your relationship issues.

If it's so all-fired painful for you, RSVP no and be away (e. g. a girls' weekend or whatever, but be out of town). Your partner can go stag. You do trust him out of your sight, yes?

Then either go to the wedding with him and smile and remember that it's all about the bride and groom and not you and your problems, or don't go. Don't be in between about this, pining for a happiness like the bride and groom's (or whatever you feel is going to set you off while there). If you cannot go and be supportive to the couple, then don't go.

Assuming he did not cheat on you with the bride or one of the bridesmaids, then yes, you are overreacting. Go to the wedding. Wish the bride and groom well.