I'm in a similar situation -- no real advice but just to say I know what you're going through and its absolute hell.
My situation is marginally easier as my H did agree to move out and we are now living separately and he accepts this.
But he's still very volatile and erratic, alternating between being unpleasant to the point of being abusive and then being totally dependent on me -- emotionally and financially - and turning up late at night and crying and shouting at me for hours when I'm trying to work/deal with my child etc. It's incredibly draining. He's made a number of not very serious suicide attempts and has gone through periods of enormous financial stress so I have had to bail him out with his rent and other bills on several occasions.
The guilt is enormous.
Ultimately, though, you can't save someone, especially if they refuse to accept help offered to them and face up to their illness. You do have some responsibilities to him but these should not include sacrificing yourself. He has no right to force you into a position of being his carer against your will when you have effectively checked out of the marriage. Remember, also, that your children need one sane, stable parent who can put their needs above all else. Looking after them, and yourself as their primary carer, takes priority over looking after someone who is essentially refusing to address their own problems.
I wish I could offer you a quick fix or advice on the practicalities but not having gone through this I can't, really - I'm sure others will be along who can but I wonder if Women's Aid might be a good starting point? I am still living with this on a daily basis and its affected my MH as well.