There is no one standout answer. You could talk with her and share with her your feelings, but I think my advice to you is just take a vacation away from her for a while. I can understand that you're a few days removed from her and all these feelings are brewing inside you, but give it a good think, give yourself some time to reflect, some time to breathe. I'm not saying isolate yourself from the world in the process, but you just have to take life with stride. You're young and you're off to that wonderful world called college where you'll explore more about yourself, more about the world, more about other people, and you'll meet others, some who may pitch in with a different perspective about life. I know that whole thing about college sounds cliche, but it's true.
Reading your story reminds me of high school. I was in a similar situation. In my junior year I fell in love with a senior girl, who was in a relationship. She fell for me too, but it was a confusing time, and one thing led to another, and she was out of my life and moved on to college. That very summer, I couldn't get her out of my head too and it affected me in almost everything I did. But hey, I've always believed time heals all wounds, and I got through it somehow and you will too. I had a wonderful senior year and after thinking it through, I decided to go to college with this girl and somewhere down the line, we got into a relationship, but it was a rocky one. So, sometimes things don't pan out and sometimes they do.
I'm sure we can all relate to what you're going through in some way, but right now, I think you should think ahead. If fate has it that you're supposed to get together with this girl, fine, but let things take its course. But just give yourself some time to breathe, reflect, and yes, live.