I’m so sorry that you are having such a hard time. By trying so hard to be his kids’ friend, he forgot that his job is to be a parent. These children needed him to set loving limits and to help them adjust to a life with two households. Instead, he has allowed them to become tyrants and his ex to have far too much power in your marriage. He feels bad about it but is totally frustrated and overwhelmed by the situation. The result is that the kids, his ex, and now your husband are all abusing you. Of course it hurts. Of course you think about leaving.
It’s time to have a very serious talk with your husband – not a fight, a talk. Remind him that this situation isn’t good for anyone. Ask him to go with you to a counselor who specializes in stepfamilies in a last effort to try to save your marriage. You two can’t do this alone or with a book. You need to be talking to someone who can help the two of you come up with solutions that fit your unique situation and who can support you while you make some very difficult changes.
I wish you well.