Good luck with that :-) We cannot control our feelings, or thoughts, just our actions. Our feelings let us know what is going on, and I'm glad because that caused you to act by writing this post! Think of the other things you could have done? You could have acted on your anger and feeling of being patronized but you decided to check all that out, instead, to see what other people's take on it might be. That's wonderful! That's what you are supposed to do! Your background experiences have led to your thoughts and feelings coming up and here's an opportunity to work with them and, hopefully, next time a situation like this comes up, you remember this one and its resolution you decided on so the next situation doesn't get as uncomfortable.

I was with you until your friend innocently wondered if you were going, as you all had not discussed the event before I gather, and your imagination took over and took that as an invitation. You already got the invitation in the email, like everyone else did and you decided you did not want to go! Now your friend is going and associating with other people you don't know but don't think you want to associate with but it sounds like your "don't like groups" might hide a bit of social anxiety? So, you are stuck between the rock and the hard place, kind of wanting to be included (you were, you got the email like everyone else) but being scared/feeling awkward and unwanted and not liking that feeling so you have to use your imagination and thoughts to stir it up and confuse things :-)

It helps me if I refuse to think about the other people and what they are thinking/looking like/doing, etc. and admit to myself, "I don't know these people and I'm afraid they won't like me". I can only work on my own fears and test out my own thoughts to see how true or false they are, can only act the way I want. Deciding someone else is doing something to mess me up (planning to leave at a certain time when I can't or won't go) gives me all the "power" (Why would they be thinking about me? They have their own plans and things they are doing for themselves, they aren't thinking about me, too much coordination would be needed by too many people and they don't know me to be concerned about me and what I might or might not do?) and also distracts me from useful thoughts about my own actions. If I am thinking about what other people are doing, I cannot also be thinking about myself first and I'm the only person "here", the only person that counts in my life.

It is all right to be too uncomfortable to want to go do some activity with strangers one is not sure one will like/get along with but starting from that point and not weaving a whole different, confusing story in there can save some additional angst with one's self.