He can't forgive himself

I need some serious help. My ex-fiance(who is in the marines) and i have been broken up for awhile, but he keeps coming back to me and asking my back out only to change his mind a few days later. (This is not about how i can get him back). He is a really great guy but while we were broken up he did somethings that i don't think he can forgive himself for and as some one who knows him very well i can tell he is incredibly unhappy. he drinks all the time now and has openly stated to me that he isn't happy and that he just doesn't care about himself anymore or what happens to him. it's like he can't forgive himself and is punishing himself, he also said he is afraid i could never forgive him but i know all the things he has done and if he actually made the effort to win me back and showed me that he could learn from his mistakes i could forgive him, but instead of trying he gets so defensive and mean. I love him a lot and i really want him to be happy and healthy, he truly is my best friend. I have the hope that once he can forgive himself and gets to a happier place that he will realize that he needs me and do what he needs to do to win me back, but it is only a hope not an expectation. my main concern is that he finds happiness and I don't know how to help him get out of this downward spiral. The only reason i am this invested in helping him is because he keeps coming back to me and can't seem to walk away from us. I really don't know how to help though, i have tried sticking by his side through it but he isn't nice to me and i have tried to get him to see that he is worth it and deserves forgiveness and happiness but i know i can't force him to see that. I just don't know what to do anymore, i think i need to not talk to him and maybe missing me will push him to start getting out of this rut and finding happiness but i don't know and could use some help!!