I am pretty much in the same situation with a guy who already has a girlfriend, in fact, I have had this situation every single time I have been interested in someone (be they with a girlfriend, or just not interested in me, or gay). What is difficult in this situation (which you may feel too) is that your feelings are not acknowledged, and are worth jack **** to anyone else because you can't act upon them, but if you can acknowledge them yourself, and let yourself compartmentalise this feeling, you can start to let go of the wish to be with her, or wondering whether she could be with you, which is the most damaging thing. Of course you may be surprised and she ends up going for you after dumping her boyfriend, but it's probably the worst thing for you to hope for this and have your hopes smashed. If you can, try and devote yourself to something else you are passionate about, and focus your energies upon that rather than the negative energy that one inevitably generates by hoping for an unlikely cause.