Well, this is definitely a sticky situation, actually, a very similar one to the one I was in when I was about 14-15. I really like my best friend, even thought I may love her. She never had a boyfriend though and we were so close. She was very "touchy-feely" as well and all I wanted to do was be with her, but I was so scared because we went to a very accepting school of many gay and lesbian students but she was a self proclaimed straight person. So I was terrified it would ruin our relationship. So I pretended to be a boy on the internet and she started talking to me about 'him' and how great he was and how much she liked him and it made me feel so good knowing that that boy was me. I wasn't pretending anything else, I was just being me and I thought if I told her how I felt and confessed that I liked her so much I wanted her to see me past my gender but it just kept going. I never had the guts to admit to her who I was until one day I just made him fall off the face of the earth (that was nearly a year and a half later) after I got a girlfriend when I turned 17 (whom I am still happily with!!! We're engaged to be married!!!<33) She became very over jealous like I was cheating on her or something and when I confronted her about it she said that I just dropped her and didn't care about her and it hurt my heart more than anything because I had had feelings for her a few years before that and I wanted her to know how much I really loved her, but I gave most of my attention to my brand new girlfriend and every thing just fell apart in the end. It was alot of pain and a lot of being closeted (my friend never new I was a lesbian and I didn't even have the courage to let out the 'bi-sexual' card until after I was 16)
My suggestion: If it were me, knowing what I know now, I would say to you, if your feelings for her are really strong, be patient. Wait for her until she is ready and of course no longer in a relationship. You don't want her to think you are trying to sabotage her in the name of jealousy. Just stick by her side and just care for her unconditionally and when you feel the time is right, gently let her into your heart. Even if the answer is still 'impossible' at least your mind will be at ease. At least then you will know that you gave it a chance. And if she isn't willing to be with you, just know that God has a better person waiting for you. At 15 you still have plenty of time to meet your true love, and people who reject or abuse you really just aren't worth the heartache.
I hope I have been of some help! <3
~.^
-Rhea