Maybe I can offer some advice. I tend to look at things several ways, so I apologize if I perhaps give too much advice.
To start out, I think you should move on. At least for now. You SHOULDN'T try to separate them just because you might have a little history. Why? Because when she says NO to you, you damage future attempts, and possibly a good friendship. Many relationships your age do not last. Maybe a few years, but most of the time things gradually die off. You most likely will have an opportunity later on to pursue her. By moving on and trying to date other people, you are also gaining valuable experience while also resolving the crushing feelings you have for her.
From personal experience, banking on just one person, no matter how awesome or "perfect" they seem, is a REALLY BAD IDEA. Shit happens, and love and relationships are very conditional to time, place, and circumstances. Even if you like them, they might not be in a good place to accept your feelings, and you are ultimately risking something that is very important to you, for no gain.
Realistically, your feelings will fade. I'm still crazy about my ex, and I think about her nearly every day. It's going to suck, I know. But you honestly need to calm down, vent to somebody close, hang out with other friends, and start trying to date other people in the meantime. Your feelings might not change significantly (like mine), but at least they will become manageable if you can at least make progress somewhere else in your social life.
MOST IMPORTANTLY, I think you need to cut contact with them. People might argue with me on this one, but I think that would be best in order to truly resolve those feelings you have for her (to a manageable state). Perhaps if you want to give her a reason and make this a formal event, you could explain your feelings to her (and maybe her boyfriend). I suggest not putting her in a position where you make her feel obligated to leave her boyfriend for you; this is a bad idea, and it will almost certainly backfire on you. You really put her in a situation where she has to choose. Because she probably won't choose you (which, given her circumstances, is very fair). And your image to her becomes a lot less; desperation comes off as creepiness, which is a major turn-off.
Instead, if you were like "so, I've been struggling with my feelings with you. I know you are in a relationship, so I think I need to give myself some space from you and your boyfriend for a while. I like you, but I'm going to eventually do something to make you hate me, which I could not bear. If you ever become single, you should hit me up and we can grab some food/coffee!". Something to that extent.
I tend to give very detailed advice. I apologize for that. I hope I covered everything.