I feel that you should try and socialise a bit more and meet new people. I think this girl you are close friends with may be using you a little bit. You say that you do things for her that no one else will. If she has a proper relationship with her boyfriend then she is just playing with you. You would be better off finding a girl that is single and who you can have a proper relationship with. The way she is behaving towards you isn't that of a normal friendship. She is definitely either very confused in her feelings towards you or she is just flirting. You could try talking to her and ask her how she really feels towards you.