He's normally reserved, but he has opened up this time.
He has told you that you are a burden to him, that when he has too much to handle, he needs anyone but you. Believe him.
He has a lot of family issues (mom, deceased father, a child, unsupportive siblings) and you two have no time together. You have no place to be together, yet you don't say how you are working toward that. You are "talking about a house and marriage" yet you cannot talk to each other anger and hurt. You say you are best friends. How is that? When he's angry, he hurts his best friend? And in front of the office.
You love him, I get it. But you don't have a friendship or a "great relationship." You have sex and not much else. When you have fights, you spend the night apart and don't really work anything out. He pushes you away and your mission is to not be a stressor. You're not his best friend, L.. Best friends run to each other in times of trouble, lean on each other. They don't hold back and hope they won't piss the other one off.
You have the added problem of working in the same place. Whichever one of you is junior to the other needs to get a resume together and look for a better workplace. Please get some counseling to figure out what you saw in this man and how you can recognize the signs of disaster in an angry man in the future. You cannot fix him if he doesn't want to be fixed. You cannot ignore it and hope he will magically change before the next time.