When this girl tell you she loves you, are you getting the feeling that she means it in a friendship way, or do you feel that she is actually in love with you?
If the latter, she does need to decide what she wants. If I were you, I would have a serious talk with her. Tell her exactly how you feel and tell her that it is really hurting you to have these feelings, but not be able to have her.
Tell her that if she decides to stay with her boyfriend then you really will have to move on, as this love triangle is no good for any of you.
If she still stays with him, cut communication with her for a while. Hopefully, she will miss you so much that she will decide it is you she really needs.
There is a chance that she has no intention of leaving him, but is enjoying the attention that she is getting from you. If that is true, she has no right to play games with your feelings, and you really do need to move on.
Hopefully, a serious talk will help you decide what is best. If she is a player, then it is time to find someone who will love and respect you, but hopefully she shares your feelings and will leave her boyfriend for you.
Good luck, I hope it works out for you.