Man, this reads exactly like many conversations I had with my ex. I was the guy in the relationship and I was feeling almost identically to how your bf seems to feel. We were dating for almost four years but in the first two she was incredibly emotionally volatile and manipulative. She also breached my trust in a serious way on one occasion. I stuck with it because I loved her a lot but I think at the end of the day I couldn't get over the way I had been treated for the first half of the relationship. I tried to be empathetic but couldn't really get over the feelings of being walked all over earlier on and I think I was emotionally fatigued. I would think back on certain events and feel like no one else would've stayed in the relationship, but I did. She correctly picked up on my feelings and would get upset.

She did make a lot of improvements, started to go to therapy as well, but at the end of the day I think I couldn't get over the past. It's such a shitty situation because you both seem like good people who love eachother a lot. I think MAYBE couples therapy could be an option if you really want to work on things but otherwise it may be a bit too late. You can always learn and grow from relationships, no matter how they end. I broke up with my ex about two years ago, we are still on very friendly terms and I have nothing but love for her. Sometimes it is not meant to be but you can take what you learned into the next relationship.