Need Help, No answers. Is it a good idea to get with her?

So I started hanging out with this girl at the start of Christmas break. I'm a junior and she's a senior. She always joked around and was over the top flirtatious and I just thought she was joking. She asked me to hang out and then the night we hung out, she kissed me. Problem is she already has a boyfriend across town but she only sees him two or three times a month. We hung out everyday over the christmas break and she drives me to school everyday now. I can't tell if she and her boyfriend actually love each other or if their relationship is just for show. We slept together and recently she got mad at me for not being completely open with her because I wouldn't tell her why I was sad or about how I felt. She really cares about how I am and how I feel. She always says I'm cute and funny and she makes me not feel so lonely. I think I'm gonna to ask her out. She started this relationship, and when I was kind of wary about making moves on her (because I didn't want to be rude and overstep boundaries) she would encourage me to be bolder. She wants to be emotionally and physically intimate, so I think she's looking for a relationship with me and got with this other guy just so she was with someone. It seems to me that she wants me to ask her out so she can dump him and be with me. I've helped a lot of people with their relationships but since this one is with me, I don't how clouded my judgement is. Do you think my logic is pretty sound or have I completely missed the mark?