Home wrecker

I am 21 years old and have reached what I believe is the biggest obstacle in my life. I'm deeply in love with this girl who I share a great history with. We met in college a few years ago and I had feelings for her which i never expressed because I never made a move and she eventually got a boyfriend a month or so after knowing her. They went out for awhile but all the while we were best friends, we hung out alot in school and helped each other. Months went by and she eventually started telling me her relationship problems. I found out that he went a-wall and was using her home to evade the "government agents", he'd mooch off her, and constantly manipulate her to get what he wanted. On several occasions she would see him on dating sites but he'd always have an excuse. Shes very easily turned by others and tends to lean towards what makes them happy and not herself. However, for some reason she lacks that habit with me, shes completely herself. After a few more months of evasion they finally caught him and he had to do time and on top of that serve his country, which honestly was fortunate for him because they could of done worse. We went out a few days later after the incident and I told her how I felt because she'd constantly try to hook me up with another girl and I finally got tired of it and cracked. We went somewhere after that, one thing led to another and I'm sure you can use your imagination after that. We did this alot and it was guilt for both of us because she still hadn't ended things with him even though she had wanted to. He returned a few months later, she eventually broke up with him, but she still didn't choose to be with me...she said she needed time. I eventually stopped talking to her because it hurt seeing her and didn't help that we went to the same school so it was inevitable that I'd see her again..and again. She started going out with someone else and then I felt extremely emotionally struck because I felt betrayed. I coped with it because I knew what I had done was wrong, but I did it because she was so "dead" that she would of never left him if I hadn't. It had been a year to today that we had no communication at all that she started talking to me again just last month. I only found out things about her through friends, thats how I found out she had a new bf, she was till with him when she contacted me. She apologized for everything and we made amends...but now were back to how it was. Shes a honest drunk, and she told me she was breaking up with her current bf because she felt that he wasn't contributing to their relationship and only with her for the "perks", among other things. I didn't want her to go home along drunk so I offered her a ride, we talked alot more and she invited me in because I live a good way from her and it was 3 am...you can imagine the rest. But before we did she made a promise that wed have a clean slate, because we loved each other so much..we even said so when we looked in each others eyes and I can say honestly I never felt so overwhelmed and happy in my entire life that night when she said those three words to me..Well..she didn't completely break up with him because he doesn't want to let her go, so their taking a "break". He hasn't told him what we did, but she said in a weeks time she will have made up her mind on everything, who she is, what she wants, etc. This was Friday May 21, 2010. We aren't allowed to talk/see each other but she already called me today telling me that she was going to tell him. But I told her it wouldn't be wise cause he'd most likely tell her family and I would never be able to face them again, and I'm close to them. She quickly changed her mind, proceeded to ask me if I were mad at her. I told her no I'm mad at myself for being a home wrecker and for making her do a promise that she didn't seem confident to keep, that promise being able to break up with him completely. So now I'm here, feeling alone in the world because the woman I love and hold the closest to my heart may be lost. I posted this because I wanted to get it off my chest, and hear others opinions because I'm completely lost...