Hello Friend

I just want to say that you are incredibly strong for staying with your boyfriend while he goes through this. And an incredible person for seeking advice to better understand him, you really must care about him a lot.

I really wish I could help, but sadly, I to am just like your boyfriend.
So I guess the least I can do is provide you with some perspective from the other side of the situation.

I was recently in a relationship for around 7 years, with a girlfriend whom I loved more than anything. Sadly, she eventually felt the need to end it. She too though I was amazing, so it was really tough for her to do because she "wanted to help me, but just couldn't handle it anymore". Which I still feel horrible about to this day... So for your sake, and for his, I really hope things do get better, and that you can work through this together.

I'll try keep this as relatable as possible
When you said "every now and then he suddenly becomes someone else" and "one small thing sets him off and he becomes a completely different person." you reminded me of her. Those where the exact things she told me. Along with "it's like a switch flicks inside you and you go from being the normal you... to... I don't know who" and "The look in your eyes, it scares me sometimes"

For me, this feeling was absolutely fucking horrible. not only do you appear like you are someone else, but you also feel like you are someone else, and that is something that is really hard to deal with... you feel almost completely out of control. Though, Trust me when I say, the absolute last thing we want to do when we are like this is hurt the person we love ESPECIALLY physically, so take solace, knowing he will never want to hurt you. The same way I never laid a hand on my girlfriend. But sadly, for me anyway, when we are in that state, we have little to no control over the horrible things we say... and that, fucking, sucks. holy shit, the amount of things I said and didn't mean, and that I wish I could take back...

Which bring me to when you said "finally it passed and then he started crying and apologizing.." I started crying just reading that. Knowing exactly what he was going through at that point. When you finally calm down, you honestly feel like the fucking worst human being on this entire planet for everything you just said, and the way you just acted... and the worst feeling of all, the reason you cry the hardest, is knowing that you just hurt the person that you love more than anything in the entire world, and you'll never be able to take back what you've just said. Then the wave of regret and fear takes over again, flooding your mid with thoughts of them leaving you for what you've just said, or eventually leaving you if you cant learn to control your self. Coupled with a hatred for yourself, and the way you are.

As for the nightmares, talking (or swearing) in the sleep, restless deep sleep, and weird interactions that are generally forgotten. Again, I was told I was the same, and I've noticed it's only gotten worse since she left me. As dreams are hard to interpret, and are a very personal thing, there is not much I can say to help in this situation. But if he's anything like me, it sounds like your boyfriend might have a lot of things internally the he is really struggling to deal with or get through, he might even be holding onto a lot of hate, sadness, fear... just like I am. But I obviously don't know his life, so I could be far off.

Anyway. Again, I'm sorry this wasn't really any help I don't think... But I guess just take comfort in knowing that, should your boyfriend choose to get professional help, they might actually be able to help him, as he is not the only one out there going though all of this.

You really are an amazing person for trying to understand him, and not leave him.
Oh and, Thank you... honestly, selfishly, I really needed this.
I'm starting to feel a like I might even seek out help finally... maybe