Dear K.,
Well, I stayed in my similar marriage, and finally left just before I went crazy. Of course, you already know, life is always hard, but mine did get a bit easier when I left my husband. He was really killing my spirit. I finally got a divorce and remarried. I have had a much happier life, although it has been no easy road. ....adult step children can be really awful. So, as mean as it sounds, I say yes, go ahead and separate. Maybe he will be anxious to make things better, or maybe he is waiting for you to do something.
But, first, try to talk and communicate. I know, he probably won't discuss anything. But, do try, and get counseling and pray a lot and just listen for the solution to come to you. You have waited for a long time, just devote a bit more time, so that you will have no regrets. ....having no regrets is the key to having a more stress free life ahead, and it will only take a little while, until you get the right answer.
Good Luck, C. N.