Girl(friend?) problems... Looking for advice

Alright... I'm 24m and I haven't ever really been a relationship guy before but I've met a girl at my university who really matches my "blueprint" for what I like in a person... I went against the usual initial feelings of avoiding anything serious and decided "What the hell, I'm going give this an honest try.. Not gonna hold back." ... To say the least, this has been quite a complicated ride full of mixed emotions and confusion... Sorry if this ends up being a long entry but I guess if someone is willing to read the whole thing they may also care to give some advice as to how I should handle this.. I will start with the fact that she is almost 5 years younger than I, but she is mature beyond her years as far as how she communicates and how she handles certain situations.. We started out as study buddies and that was going great. The first three weeks of knowing eachother we spent almost every waking hour together renting private study rooms in the library (the studying usually turned into hours of talking and laughing).. We really do have a lot in common and it's easy for us to lose track of time when we are with eachother. I also took her out to eat at a few cafes which it felt good to be able to do that without things getting weird at all. I could tell pretty quickly that she was very into me, but I continued to play it cool. one night while in a study room she was being very open about her feelings toward me and I felt like going in for a kiss... So I did... And Denied!! She shyly backed away and told me she hasn't told me everything... Everything became super slow mo as I racked my brain... I asked the first question that popped into my head "You have a boyfriend???!!" This is where I think a guy in his right mind would have run but I was so intrigued by her.. So She was in a 3 year relationship and currently living with the guy! She insisted that she would never lead someone on like she did to me but that she really liked me and that she hasn't felt as good as she does when she's with me in a long time. She stressed that her relationship has been on the rocks for a long time and she was ready to move on.. While this frustrated me initially, I couldn't help but respect her for the honesty and that she wouldn't let me kiss her because she was actively in a relationship.. That's cool. She wanted to continue to study with me and I was totally cool with that.. I was going to put my feelings for her on the side and just be friends with her.. I talked with her for hours and tried to be unbiased with my advice to her on how her situation should be handled. Im not sure what I was thinking but I continued to counsel her.. I often would find myself telling her to try to work things out with him and if it hits rock bottom then that's the time to get out-- however every time I took a neutral stance she seemed to want to express her attraction toward me. (We kissed a few times at this point; nothing serious) But it gets a little fuzzy for me, the transition between me being a friendly support system and things getting serious between us... she broke up with him which was great.. She stayed with me most nights and we got along great and grew very close. THE RED FLAGS: Ok briefly, the first negative sign (which I passively let go) is the fact that she went back to her BF and slept with him; I believe they were still dating at this point (this was within the first couple of weeks of me knowing her; when she was trying to decide whether or not to leave him for me).. Fast forward more than a month... I have been unwaveringly honest with her at every instance.. I will do anything for her and she knows it. Up to this point I have been up and down with my trust in her. -she stays at her place and claims to sleep on the futon while her EX sleeps on the bed.. She says it feels like home and she misses being at her apartment. -to this day she texts him everyday (as far as I know, it's constant) she claims that he is really having a hard time and she is too caring just to let him struggle-- personally I am sick of her going over there to have late night "talks" with him. - one night, recently, I sat her down and gave her the ultimatum which should be implied. I wasn't asking her to move in with me I was just telling her that if she really wants to be with me she needs to make a commitment to being more resourceful and finding alternate living arrangements.. No go. She hasn't even attempted this. - she has told me face to face that she's feeling pulled back to him sometimes because she knows she has security with him (which makes me a risk in her eyes) I tell her that that's a sign that we aren't doing the right thing by being together but she won't make a decision!!!!!! - when she's with me it's more passionate than anything I've experienced but she also talks about her ex around me-- I've told her it makes me more distant when I know they are communicating and still sleeping in the same house!!! -she still wants to be friends with him -I feel like a rebound....:/ -she's not willing to commit and she losing me fast Dang, just writing this I know what the answer is. This is unhealthy as hell right? She has half of her things at my place right now.. And it would suck if I ended this and she was being honest the whole time..I'm sure I could get more detailed but this is exhausting.. Lay it on me peeps