I think because this is your "first official girlfriend" it might be hard to realize the relationship may have run it's course.
It sounds like there may be a lot more things that you dislike about this girl than you like about her - she isn't going to change these behaviors my friend - this is how she is and since you don't see a future with her, do the right thing for both of you and end it nicely.
you sound intelligent and know what you don't like in a woman now, so keep that in mind when you start dating. Just know that she will badmouth you like she does everyone else when you end it.....she sounds very immature and insecure. you can do better.

Good luck!