happykay.
The children ...
You see, that is why so many of us stay. We're not idiots. We're normal, sentient human beings, who are being eroded, inch by inch, and are having our boundaries of what is acceptable moved, little by little. But, by and large, we understand that, and we resist that. Hence, I think, why so many of us end up with "mental health issues".
The children is what keeps us there. Obviously, it should work for the abuser as well. he should be thinking: "What am I doing to my children? I must stop!"
But he doesn't.
And if you start thinking about the implications of all that, it gets vey awful.
The simple answer - which is also the complex answer - is what other posters have already said: if you leave, you are actually doing your children a favour. The current situation is teaching your children bad things about relationships. However much you think you are protecting them, you aren't. You can't. It's not possible.
As to the situation with your step-child - it is awful. But in a way, all you can do is accept your lack of control there, too. This is not what you chose. You chose, you acted for, you did everything you could, to create a happy home, a safe place for you, the children and, yes, your husband.
Your husband does not want this. What he wants, above anything else, is control of another human being - in this case, you. Though if he really, really cared about you, as you, he wouldn't behave like this.
He really doesn't care enough about the children to stop.
I write all the above feeling as though I am writing to myself. I still am "held hostage" by my desire to give my children what I so much want them to have.
I keep telling myself that it is only me that he is so awful to, and that it just doesn't matter - I can take it. I tell myself that if I can just cope, I can hide everything from the children and give them this happy family and childhood that I am so desperate for them to have.
But I am slowly coming to believe that it is already not there, and that it is wrong to collude with someone so sick that they will use this (very human) love for our children to exert a horrible, cruel pressure on me.
I think, honestly, that you and I need to learn not to be held hostage by our love for the children and our optimism/hope. I think we have to let our dreams/wishes go, here, and make the best of what is.