You should give up on her because it doesn't matter that she likes you. If she's staying with her boyfriend then you're just someone that gives her attention and she keeps it going by playing with your emotions. If she REALLY wanted to be with you then she would leave the boyfriend and she would be with you knowing that you like her.. you're a sure thing.
Stop fawning over her. Don't give her anymore of your attention, start scouting out other girls that interest you and leave her to her boyfriend. You're wasting your time and your emotions on her as long as she's with him. Besides, you give her absolutely NO reason to break up with him while you're giving her all this attention while with him.
You deserve to have a better girl then someone like her that flirts and gives another guy her attention while in a relationship with someone else.
Don't "tell her to break up with her boyfriend" that is controlling behaviour. You might consider telling her that you are unable to continue the friendship due to your feelings for her and then let her decide who she wants to be with after that.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion