I like a girl with a boyfriend, what should I do?

Well, I've known her for a year, and we met before it's just this past year we really got to know each other with multiple classes together. It wasn't until a couple months ago that I liked her. The problem was that she has a boyfriend for 3 years, and I've known this and it was initially the reason why I didn't have feelings for her. One night I changed my opinion and wanted to pursue her. I started texting her and genuinely started making her feel special. I told her I liked her and she said she had feelings for me, but had a boyfriend. I know it was wrong but I started being aggressive, having intimate contact, talking on the phone, constantly texting, and doing stuff alone together. She told me that she admired my ambition and goals in life, and that she disliked the passive attitude and the immaturity displayed in her boyfriend. She felt guilty and told her boyfriend and that was supposed to be it, but we came back to each other. Then things started getting really serious and we did more than that, but not everything. So I became the other man while her and her boyfriend had quarrels over me, and I gave her an ultimatum. She asked me before her decision if I would wait and just be her friend if she broke up with her bf, so she could get over him. I paused to think about losing our intimacy, and then I told her I think I would. She chose him, because she felt that she could cheat if we we're together, and she thinks I would hold it against her if she did. She also felt that it would be wise to stay with someone who loves her rather than one with feelings, and told me that she thought it was just an infatuation. She also said she doesn't want me to leave her life. Nowadays we haven't crossed the line, but we still have classes together and we're literally together almost always. We look like a couple, but we're not. We both said we still have feelings, but she still doesn't want to leave her bf. She told me nonchalant that she doesn't love her boyfriend anymore, but still wants to work it out. I care for her so much, and felt a lot, most of the time when we kissed. She said she didn't feel that spark when we did, yet I feel like she was feeling guilt when we did. I honestly see something long term with her and she's been like a best friend ever since we got really close. I don't see how we're so compatible (she says I'm the fastest that she's ever liked any guy, because she usually plays hard to get) yet she stays with her bf, and I don't want her to leave my life, but it hurts knowing. We still hang out alone together, her boyfriend knows that we still have classes together and are still friends. Should I stay friends with her and look to the future if she breaks up with him? Does she think she's not good enough for me because of what she's done with me when she was with her bf, and that she doesn't deserve me? Does she keep coming back because I give her so much attention? Ladies that have gone through this situation help me out. I am not a guy looking for a fling, or whatever, I want a relationship.