Noooooooooooooooooo, you are not overreacting! I would be pitching fits right now if it were me...not only over the fact that two other girls who [from the impression I get] weren't as close with the bride were chosen as bridesmaids while you weren't, but also that
no one deigned to tell you except via a snotty little email. Are you sure they're out of high school?
As far as the wedding is concerned, I think you should go, because you might regret not going later. However, I would strongly encourage you to say something to the bride--your "friend"--because she really needs to know that you're upset and I think she's got some explaining to do. I realize it's up to the bride whom she chooses to be bridesmaids, but it seems really cold and ungrateful to overlook someone who's been there for her like you say you have.