What do I do about a girl I'm crazy about who is promiscuous?

I’m crazy about a girl, but she’s engaged to be married. I’m fine with that and I’m very happy for her. We’re both very fond of each other and in a different place and time I’d love to be crazy about her and dote on her and that sort of thing, but this is how it is. She cheats though and now I feel betrayed. I have this strange feeling (irrational, yes) that she should be satisfied that since she can’t have me ( I refused her advances) that she should go back to her fiance and be happy. But she sleeps around and it breaks my heart. She’s being horrible to herself and making me watch is hard enough, but I also feel jealous and hurt. She might sound psycho, but she’s actually trying really hard to change and that makes her even more endearing to me, which makes it harder for me, too. I need advice on this one.