Doesn't Damon always have this "i don't care" expressions? Always joking around and give impressions that nothing's matter? He's well known for that. He never let anyone knows his true feeling and always try to hide it. Only few people recognize this about Damon. I'm new to this show, but I do like it so that I watched all the episodes. He went from wanting destroy MF to want to save it. He went from not caring about someone to really care about someone, even fall in love with Elena. But he came to this point not without reason. His reason is becoz he found someone who believe that he worth saving and can be saved. This is the reason that made him want to start to bring up his humanity. Elena's love is just a bonus. when Elena's repeating the same words as Kat's when rejected him at season 2 he snapped. But that's not the case in season 3. He handle it well, means that he has new understanding. There's no indication in season 4 that he tried to seduce Elena like he used to be in the previous seasons.
Season 5 circumstances really hard for Damon. He met the ghost from his horrible past, Enzo. A friend he let to die so he could save him self. Clearly he felt guilty, and I think he wants to redeem himself to Enzo. I think he knew if he wanted to redeem himself to Enzo he would've eventually follow their old revenge plan. He was conflicted. In his mind, there are two possibilities, Elena will defend him which means he changed her to be as bad as him, or Elena will against him but since he will do it anyway it will means that he will hurt her and their relationship. And I think this is why he chose to let Elena go (break up) and I think this is where his words "I refuse to change you" came from.
His speech to Elena, in 5x13, basically saying that he was wrong for having a thought that Elena will get influenced by his dark side and make her change to a bad person like him. He was underestimating Elena's good quality as a person. And he needs her to be his good influence. Kat actually didn't understand what he's talking about. Her respond confused Damon becoz she talk about her worries of him lashing out when he didn't get what he wants. Kat's respond is a common respond from people who doesn't know him well. Common respond from people who always think that Damon cannot be saved. This what break his heart, Elena lose her faith that he can be saved. Her line: It's about you... about the person I can't change.. the person you really are. That person was right to let me go. basically comfirmed his fear that he is a bad influence for Elena, and that she admit that he would change her become worst. He's a bad influence becoz he just bad. Damon translate this as now Elena really thinks that he is a monster without redeeming quality just like everybody does. becoz Elena he knows is someone who never lose hope, so when Elena lose hope in him, he believe he's hopeless. So then with this belief, Damon redeem himself to Enzo by proceeding the revenge plan Enzo offered in a monstrous way, in Enzo's way. At least he could redeem himself to someone who once meaningful to him. Honestly, I believe his revenge plan has nothing to do with his pain. He will do that for Enzo no matter what. maybe, if Elena's there, he will do it not in such extreme way. But since there's only Enzo, so yeah Enzo's way rules. LOL
So to the question if Damon can forgive himself, I'd say he would when he can see that there's someone still have faith in him genuinely. But I don't know if he could if somewhere along the show the writer decide to add more obstacle to his redemption by making him hurting Elena (maybe feeding from her) to the point of her death (or almost death)