Been there done that- on the flip side of your situation... My GF was a year younger & I went off to college. She was sure that I was the one, and I just couldn't wait to go to college & party!!! I wasn't sure what to expect, so we tried to make it work, and TRUST ME HERE - it never works.
Long story short, It's time for you to move on & let her do her thing. With your age difference, things are about to get really weird anyhow. She's going to party & expirament with other guys weather you're dating or not, so do both of yourselves a favor & take it back a notch. You can always visit when she's home, but if you end up the jealous boyfriend from back home all of her friends will make her resent you. If she isn't ready to break it off, then keep it casual & visit her but understand that when the cat's away...
I know it's probably painfull thinking about it that way, but you need to look around a bit to make sure she's all you really think she is!!! TrUSt ME doOd, when yer married game over, so enjoy life now!!!
Tough love brother!