I wonder sometimes if I might be partially to blame for the onset of his depression.
Partly it's because of the hateful way he treated me sometimes while in his hyperactive phase. Like he couldn't stand to have me around even though he was always the one insisting for us to do everything together. I offered multiple times to leave him alone to do his thing, when he was back to normal, but he always refused.

Partly it's because of something he said during one specific anger episode. He said he "needed space" because while at work he had to be nice to customers, and at home he had to be nice to me, and he just wanted to be around strangers he didn't have to care or worry about and could be rude to if he wanted. In that moment, he said that every nice gesture done towards me was basically out of obligation and he found it taxing, like coming to check on me while out or holding my hand.
I tried to tell him that all these "expectations" he felt he had to comply to were all in his head as I never demanded any of it from him, but it was like talking to a wall.
The next day, when he was calm, I asked him if, since everything seemed to be an obligation, there was anything at all he enjoyed about being in a relationship. He said he had overreacted (his go to phrase after every angry outburst) and purposefully exaggerated things, and that he just had "issues to be with someone" and it happened in each of his relationships. I then told him that since I loved him, I wanted him to be happy more than I wanted to be with him and that if being with me was making him unhappy, I would leave at once. He told me to stay and, when I asked what I could do to improve the situation, he said I was doing everything perfectly and the issues were all his.

This is the same refrain he brought up after he crashed. That I was the perfect girlfriend and did everything right but he just needed to "be alone in his head" and think only about himself without worrying about anyone else's needs. In vain I tried to explain again that we are two different independent individuals and I never asked him to "think for two" - I suspect it's not something he can consciously change.

I have moments when I think maybe I was just making him miserable as he didn't enjoy my company anymore, but then it would make no sense for him to spend the following week glued to my side, even taking a trip together and having a blast. When I asked, he said that even though we were always together he was "alone in his head". Or why he would tell me he'd have kept spending all the time with me if I'd stayed as I'm the best company, or that it would be weird to stay there without me, or how much he'd miss me after I left.

I'm not sure i understand. It would seem my company is indeed enjoyable to him, it's the pressure of whatever "being in a relationship" means in his head that he can't handle. And as much as I tried to "logic it away", I don't think that approach could ever work.

I once again repeated that I just wanted him to be happy and if being without me was what he needed, I'd gladly leave. He snorted, looking miserable, and said nothing would make him happy.

Sorry for rambling - I'm just thinking out loud. I have no idea if this can be a trait of depression or anything else or if anyone can recognise a similar pattern in their own experience, but I foud it helpful to write about.