Vagrant. Please get a grip. This girl is 20 years old. People don't know their head from their assh*le at that age...you are not dealing with an emotionally mature individual and you cannot change the way she behaves.
You are not married to this person...you haven't been exclusive to her....and you are already investing too much time into her. She is not capable of giving back to you what you give to her.
Here's what you should do....and you'll do this starting today:
Be different from all of those AFC chumps who text her constantly, "like" everything she does on Facebook, retweet her crap. Simply go ghost for awhile. She'll wonder why you are so different from the rest of the Betas who follow her around like lost dogs.
You've already f*cked her...do you know what that means? You are in pretty good standing. Girls that age will go absolutely crazy over a guy who bangs them and just ignores them afterwards. So that's what you are going to do...ignore her and watch her spin out of control. You are regaining the power that she has over you. And guess what? Women LOVE Power.
Scarcity means value. People want what they can't have. Think of what she is doing to you. Whenever she hangs out with another guy, it's means you aren't hanging out with her. That's when you desire her. If she was constantly texting you all day and didn't have other options, she wouldn't be desirable, would she?
You are going to turn the tables on this girl. Become SCARCE. You said it yourself that you are the prize...now starting acting like it. In fact, get a fvcking Sharpie and write P-R-I-Z-E across your hand to remind yourself.
No more wimpering posts for now on. Come back when you start acting ALPHA with some proof.