It sounds like you both struggle with feeling insecure. She is insecure about you in that when things got challenging for you, you ran. How can she trust you? She is most likely holding on to this other guy because that feels safer to her at this point as she has already experienced you as being a fight risk.

You get together and get intimate and your sexual intimacy has been wonderful. She WANTS to love you, yet you have never given her a chance to feel secure with you. It could be the one thing that attracts her to this other guy is probably that he is more secure about himself and she can feel he is more stable than you. It's not that he is better looking, or better in bed, or actually better than you, except he has probably been more stable. It would not surprise me if she says, "If only PotentAnarchist could be like this with me" he would be perfect because he is the one I really want". Given that she has hung on for five years now, that would be my opinion about this relationship.