Don't feel bad. When I was a senior in high school, I had a crush on a girl who I eventually found out was a freshman, so I was 17 and she was 14. I ran into her one day during my free period and we spent the whole time talking and things seemed great. I figured that would have been my best shot at a girl since I went through all of high school without getting anywhere with a girl.
Needless to say, it didn't work out. A couple of weeks later she started dating some really short dude and I saw them making out in the hallway one day. But then they broke up and she started talking to me again about how it sucked that we lived kinda far from each other and couldn't see each other during the Christmas break. So I manned up and asked the out the next day, and she told me she wasn't allowed to have real boyfriends and stuff. I managed to convince her to give me her number, saying we could go out as friends and stuff, but even after that, when I called during the break, she never picked up, or had her friends pick up and talk to me instead. It sucked realizing that I couldn't even get a freshman, and it was a big blow to my confidence, but looking back, I've realized that it wouldn't have been great even if it had worked out. She still had a lot of growing up and maturing to do, and so did I.
Not to mention, that was about 5-6 years ago and I've moved on to so many other girls. I still haven't had a girlfriend, but I look back at everything as a learning experience. I know its a cliche and easier said than done, but there really are a million other girls out there, so don't get so hung up on the one that broke your heart. You're still young and chances are the best relationships you have in your life won't come until your older and out of school, so just try to enjoy your life and do the things that make you happy without this girl.