Did you ask him to go to the other party with you? It's unclear from your post.

When he said you were included in the other invitation why did you not say "OK then, thanks, I'll change my other plans"?

Me 57/H 58
M36 S 2.5yrs R 12/13

Let me give up the need to know why things happen as they do.
I will never know and constant wondering is constant suffering.
Caroline Myss