I cant help but feeling that she really likes this guy and won't cheat on him like she's done to previous partners, including me in the past.
I wouldn't bet on it. She really liked you in the beginning to. If it does end up finally working for her, then at some point you might be able to be happy for her that she has finally found some peace. You aren't at the point yet, so all you can do is try to not let it get to you. I know it really gets to me when I think of my ex with another man, anger and pain flood my mind and body. The only way to deal with it is to accept that she is gone and let go of the feelings.