Hopegone, your first post made me laugh a bit - I had used the phrase "Debby Downer" a lot, and even "Eeyore". It seemed a perpetual black cloud hung over my house back in those days, and it is crazy-making.
I know you want to stay, and like the others have said, that is your choice. But like Anvil says, you can't reason with an unreasonable and irrational person. Part of stopping your madness is to stop trying to do that. Sure, it makes perfect and logical sense to you, but he isn't in a frame of mind to see this your way.
For me, I had to separate physically to get some clarity and peace. We kept trying for a while, but in the end, I couldn't justify staying with someone that I wanted to "change". He is who he is. I am who I am. We wanted different things in the end. It just seemed fair to let him go, in turn letting myself off the hook.
Life is peaceful these days, and it does get better! Hang in there,
~T