Don't walk...run. Run away really fast and never look back.
Personality is a trait that is well established by age 20 and she is not going to change all that much as she "matures". Believe me, those kinds of head games and communications problems only get increasingly intolerable with time and with age. You've put up with this for about 3 weeks at age 20. You'll be able to tolerate it for only about 3 seconds at age 40.
So run now before you have a history, kids and child support payments with this woman.
And while you are running, get a clue...if you get warning signs, listen to them. You do not have to be a psychologist to diagnose personality disorders. One of the best clinical tools for that is that feeling you get in your gut. Personality disordered clients tend to give counselors the same feelings they give to other people in their lives and it gives a counselor great insight into why people don't like or cannot get along with the person they are counseling. So if a woman irritates you, makes you crazy or makes you mad at the rest of the world, forget her and move on as whatever other positive qualities she has are not worth it.
A brief review...
1) An underage girl asking you to get her into bars is always a warning sign. At best she is using you, at worst she is just plain trouble.
2) Age 20 is not an excuse for being rude, pouty or incomunicative. If you ask her a straight question you deserve a straight answer. If you don't get a straight answer, move on. If anything you are doing her a favor by dumping her as there is a remote possibility of teaching her that her approach may not in fact be the best approach for snagging a guy.
3) Relationships are built on mutual trust and communication and if that is not there all the hot sex on the planet will not make it work.
4) If you are selecting a mate by looking for a hot babe, life is going to be very long and unpleasant for you. You need to find a woman with a warm and genuine smile and a willingness to use it often. And she needs to be someone you can talk to like you would a brother or your best freind. If she meets those criteria, you will almost inevitably discover kisses that melt you to your core and a sex life that transcends plain old sex.