I (24m) got involved with (20f)

There is this girl I met, I'm not really into her. We have made out a couple of times. And not to be disrespectful, but she already has a boyfriend, and some other men on the side, and then she came on to me too. So I thought, why the f**k not? I will just be someone in a long list of names. Then, I treat her with respect and show genuine appreciation, she bursts out in tears that no-one has ever treated her like this, that she wished I came earlier, that she will breakup with her boyfriend and she keeps looking at me, like it's genuinely love. But, I'm not sure, I think this will all end in a disaster. I don't think she will change, and I will have a massive heartbreak if a start developing feelings for her. I want to help her be a better human being, but I don't want to put my heart on the line for that. Any suggestions ?

TL;DR: She keeps cheating, but says she will change. Should I believe her ?