Damn dude you kinda fucked yourself on this one. She has a boyfriend, man. Forget about her until shes single again. Nobody can give you any advice here, you just gotta make a judgement call. Like decide whether you want to try to get her and ruin their relationship, possibly causing bad feelings between you and your beloved, OR... find a new girl. Or just be single for a while. Its kinda cut and dry at this point man.

My opinion? Forget about those feelings. Bury them until you almost dont recognize them. Each time you start feeling sorry for yourself because you arent with her, bitch slap yourself, then man up a little. Youre 19 dude. If its meant to be, then when shes single again and youre single again, that little spark will reignite.