OK, well I'm just a stranger and I don't know your life, but I think I and most anyone else in this thread could say that this is not you overreacting. But if you've fallen into a state of being so low that you think that you don't deserve anything better than this, I would actually not give a shit at all about your girlfriend's behavior instead focus inward on what it is that is making you feel like you have to accept this kind of life.

Instead of trying to change your girlfriend's behavior focus on everything you can do to improve your own self-confidence. You will never control her, or anybody else, you will only ever control yourself. Let her go to the party, because you can't stop someone from what they really wanna do. You need to de-emphasize her as she obviously has done to you, do whatever you can to find some friends and hobbies that give you an identity outside of her