Am I doing the right thing with this girl?

Please bear with me, this is long. Six months ago at the beginning of school this year, a really beautiful, nice girl starting hanging out with me and my friends. It started out with her just talking with us in the parking lot after school, but eventually we all started to do things together outside of school. She is the only girl in our group. I fell for her within a few weeks, and since then I have been trying to get to know her better and more personally. The problem is, she has a boyfriend of two years, and he is friends with one of my friends' brothers. She hangs out with us a lot when he is not around, so I wonder if they are really that serious. Up until now, I have been really trying to connect with her, always hanging out with her whenever I can, and trying to get closer. She always looks at me when we are talking in a group, and she smiles at me a lot and usually laughs at all of my jokes. She is shyer with me than with my other friends. She kind of flirts with my other friends, and she approaches them but seldom ever approaches me, which makes me think that she might like me a bit. Sounds pretty good right? But the fact that she has a boyfriend is really bugging me. I always told myself that it didn't matter, and that I had a shot with her. I think she is a once in a lifetime girl, so I always want to be with her. But the other day, something inside of me snapped, and I have been avoiding her like the plague ever since. The past two days, I have intentionally walked around my school waiting for all of my friends to leave the parking lot so I don't have to talk to her. I went over to my friend's house today, and she was there with her boyfriend and my friend's brother. I left five minutes later. I don't meet her eyes in the hallways like I usually do. I don't want her to all of a sudden think I'm some depressed weirdo, but I'm just tired of caring about her so much and her not knowing or returning that care. I just want to run away for ever, destroying all the progress I've made with her, but a small part of me still wants to be close to her. I'm not so deep in it that I can't reconcile my actions, but should I even bother? Please help.