To directly answer your question, it is not wrong if you decide not to take your girlfriend to a party. I had a situation come up about a month back where I got myself into what I thought was a bad situation but my gf made it easy for me.
Originally, I was invited to a party with some fellow med students. I accepted and asked if I could bring the gf and the throwers of the party said that would be great. So I go and invite the gf and she accepts.
Well, as party time neared (about 4 days before) I decided that I would feel more comfortable if I just went by myself. I told this to my gf 3 days before the party (thus what I was really doing was un-inviting her). I thought she would be upset, etc. but she had absolutely ZERO problem with it. She gave me no flack, or yelling, or anything at all. This was a party that she did want to go to and when I chickened out from bringing her along she had no problem with my late decision.
Now, my gf is in her mid 20s, is done w/ college, has a stable job, etc. She is a mature, confident, and obviously very understanding person. I have dated the 19-21 year olds and every one of them was an extremely immature pain in the arse -- generally they were a waste of my time.
Unfortunately, I feel that this is just the way things go. It simply takes some people longer to mature, get some confidence, and act like an adult. Some people never do learn, though.
The way I see it, you have three choices:
(1) Stay with your current gf and hope that she matures up a bit, stops cheating on you behind your back, and things generally will get all better again
(2) Break up and stay single. You know, to be honest with you, being single isn't all that bad. Society tries to make it look like it's a bad thing but there are a lot of advantages to being single. There's also NOTHING wrong with being single.
(3) Break up and meet a more mature woman. This is a definite possibility and a good one. I am just 2 years older than you and I am dating an older gal. Just from 6 months experience I can tell you that I'd hesitate to date a girl younger than 24 because the difference in maturity is huge!
Lastly I want to say that I think you need to get back in touch with your friends. **The right girl will want you to stay in touch with your friends** Yes, that's right. A selfish, immature girl will say "pay me attention" However, a mature woman who really cares about you and your life will want you to go out with your friends.
My advice based on the info you provided: break up, immediately and don't get back together. She's cheated on you, she's manipulated you into spending more time with her abandoning your friends, and it sounds to me like she has zero concern or respect for your personal life.
The very bright side of this story is that you are only 21. You have sooooo much more time to go out and meet other people. So go out and meet 'em.
