You can't fix other people, you can only fix yourself.
Mental illness can explain some shitty behaviors but it doesn't excuse it. You can understand where someone's coming from without condoning the decisions they've made.
but my subconscious feeling is I'm going to separate from him eventually
Mental abuse is just as damaging and having a mental illness is no excuse for abusive behaviour.
you need to think about your own life and safety
"It's like setting yourself on fire to keep the other person warm."
You are responsible for yourself, exactly where your individual flesh ends. You are not him. Ergo, you are not responsible for him. The same amount of space you afford him and carve out for him is the same amount of space you are entitled to. And it sounds like your subconscious isn't telling you what will happen. It's telling you what you want to do already.
I hope you are safe right now and that you're pulling yourself out of this situation.