A.B.
Unfortunately, your problems are those of poor planning. I'm not trying to be hard on you, but you have to know that you did not have to marry a man who had prior minor children. That situation almost never works out well for the children or the second wife.
And, if you truly wanted to stay home with your children (which is the right thing to do), then you should not have gotten pregnant again when you cannot make it work. That said, many people claim they cannot afford to stay home but there is always a way if you want it badly enough. My husband makes very little money, but we make it work because we would not have it any other way. He works two jobs so I can stay home.
I suggest you tell him he needs to make it happen by the time you have your next child. I certainly wouldn't leave him over it (even worse for the children), but try to impress upon him how strongly you feel. If he is unwilling, then you just need to accept it and not make him miserable because you are resentful. The reality is that we all choose our situations, and there is no one to blame but ourselves. Making him miserable about it will only make your children miserable.