I’m sure there must be a very sweet side to your fiance or you wouldn’t be engaged. But he is in no shape to be a married man. From what you say, he’s in serious denial that there is something fundamentally wrong with his ability to manage the world – and to be a partner. He is fortunate, indeed, that you like him better than he likes himself. But unless he takes hold of his issues, he is not going to be able to become the man he could be. He is in pain and he is scared. But for some reason he isn’t scared enough to go get an evaluation and treatment.
I do suggest that you take a big step back. A couple who fights every day is unlikely to have a good marriage. Encourage your guy to first get a thorough medical exam. There are medical conditions that could explain his feelings and behaviors. If he gets a clean bill of health, then he really should see a mental health provider for an assessment. He needn’t suffer like this. It’s unfair to ask you to suffer needlessly with him. If he continues to refuse to help himself, you may need to rethink whether a life with him is what you really want.
I wish you well.