Why does this girl let me be with her?

I met this girl in the beggining of the school year and I've been living with her for a month. I am 19 and she is 23. She has a boyfriend in China. I said I don't care I'm still interested in you something along those lines. It just came out... But... she lets me live with her and we have a good time. She was guilty in the beggining and wanted to just be friends. Then I said lets just break everything off. So we didn't talk for 3 days or so and got back together again. Everyone knows we are together all the time but we're not really gf/bf by title. We've gone all the way and her bf doesn't want to have it with her cause he promised his mom that he wouldn't till marriage.... (i don't want everyone saying wtf to this haha everyone does). She is kind of spoiled and really really really rich gets everything she wants. She doesn't think about little things like I do but I can't expect her to sometimes because she's not my girlfriend by title. Do I have the right to expect anything? I feel like we're more gf bf than her actual bf. She thinks her bf is very different than other people-- one of those wierd depressed (really emotional) artsy people. He's a photographer. She says she really should break up with him but she's afraid she can't. Her mom is helping her break up with her bf. Her bf makes her feel like she needs to feel sorry for him, but she makes me feel like I need to feel sorry for her. So I guess you can say her bf controls her and she controls me? She doesn't listen to anyone, not even her bf. She cares about others' feelings but doesn't think of them until spoken to about. But she doesn't do anything to make others feel like she cares about them. Not anyone. What's her deal? I have to make myself act like I don't care sometimes. We have a good time nevertheless hanging out and sex etc. What's wrong with me and what's wrong with her? :) If I wanted to continue this relationship after she breaks up with her bf which she is planning on this summer, how do I deal with a rich spoiled girl that doesn't think of these little things and doesn't think much about others. She's just used to it; She doesn't try to be a ***** or anything....