You can not change him, you can only change you.
You have manoeuvred yourself into a nice little spot between victim-hood and martyrdom, which gives you permission to do nothing. You say you are weak and fragile so find ways to be stronger, counselling is a very good place to start.
I don't know why he is an arse but I know why he continues to be, because you let him.
The only victims in this mess are your children because they don't have a choice, you do. You can fool yourself that you are protecting them but you aren't. By putting up with this you are teaching your children that this is OK, that its OK to be treated and to treat people like this.
He is responsible for treating you and your children badly but you are responsible for letting him. Putting up with this and staying is your choice.
No amount of us telling you that this situation is unacceptable will sway you, you have to find your own bottom.
I and the posters above me are not being unsympathetic, many of us have been in similar situations and totally understand the loneliness and the powerlessness of being treated appallingly. However, many of us have the benefit of hindsight, knowing that the real change happened when we took charge.
I think you know deep down that he won't change and the only way this situation will change, is if you change it.
P.S
The power in the ultimatums and threat lies in the probability of the follow through, it is a pointless weapon if he knows you -can't- won't follow through. Stop issuing them, because they only make you look more weak.