I just cant stand the idea that she is with happy with someone else. She broke my heart...?

Obviously you really cared for her dude! But, she doesnt care for herself and doesnt think she really deserves a good man. You will never be able to change her so do your best to try and visualize yourself over her and being with someone who knows how to appreciate your love and care. The only thing you can do for her is pray for her....she needs it.