Mate, if she has got a boyfriend then there is NO POINT going on about how you feel to her, it will just put her in an inappropriate position. If she was going to ditch her boyfriend for you she would have done.
Bide your time and carry on being her friend. She is young, right, so realistically the odds of her staying with her current boyfriend forever are slim. If she has already been having rows with him then there are some signs that there are cracks there anyway.
Allow yourself to consider the possibilities of taking an interest in other girls, even if you don't completely abandon hope of this girl you like, if she becomes single in the future, especially if she has had a bad experience with this current bf, she will be more picky about choosing someone reliable who she knows she can trust.
It is a difficult situation, really bad, I am still a little bit in this situation with one of my exes who I never wanted to split up with and now has another bf although her and me are really good mates still.
You have to just control the controllables, and her having a bf at the moment is not a factor you can control, all you can control is being the nicest possible person you can be to her, and she will remember it if a window of single-ness opens