Melanie isn't a bad person. She's a confused imperfect person as are we all, trying her best in a confusing world. Also, coming from a background where she was never unconditionally loved and accepted isn't helping
Andy *was* her rebound guy, and after a very abusive relationship, no less. Not to say that makes anything that happened okay, but considering her admission she didn't even want to have a relationship, it sounds like every day they had together was on borrowed time.
It's hard to continue to be with someone who wants to be in the relationship more than you do. It's just too bad that Melanie didn't realize that before she slept with Christo, or at the very least right after. There's no easy way to break someone's heart, but finding out they cheated on you and then just carried on like everything is fine is probably the worst.
Really, Melanie and her brother screwed up in the same way. They both got into relationships with good people that weren't quite right for them, then tried to have the life they really wanted without being honest about it with their partners.
So basically if she didn't lie and cheat but acted like the adult she's supposed to be she wouldn't feel like s@&* right now, Or hell if she didn't want to talk about how she felt she should have left andy he wasn't forcing her to stay or guilting her into the relationship