What can I do and where do I go from here?

I’m at a loss! I’ve been with my husband for 4 years, married for going on 3 years. I’m a mother to 2 adult children and stepmother to 2 teenage boys. Since the day that I met my stepsons, it’s been a constant battle to survive in the relationship with their father. The boys’ mother has taught them to hate me and to do any means possible to come between my husband and I and to try and split us up. This woman is a real piece of work. She has paraded numerous men around and has had 4 different men living with her the past 6 years and has cheated on all of them, including my husband when they were together, and she thinks that’s a perfectly fine way to behave when having children in the house. She constantly has to contact my husband, over things that have nothing to do with their children, and has to voice how my husband should be in our marriage, even telling him that if he doesn’t leave me, he’ll not see his children anymore. His oldest son is a pure replica of his mother in regards to behavior. He constantly switches girlfriends, he’s beyond rude and disrespectful to my home and myself, even my husband at times, he steals, goes through our dressers and nightstands in our bedroom, destroys things in our home, and he hit a whole new level of low when I was going through chemotherapy and he repeatedly told me he wished the cancer ate me alive and I was dead. Despite all of this, I’ve tried a number of times to show him love and extend olive branches, but he’s so spiteful, manipulative, and cruel. I’ve given up! I don’t want to be around him anymore. I get along great with the younger son, he didn’t stick to doing what their mother wanted and we have a relationship.

I just don’t know what to do. My husband will not stand up to his ex and tell her that him and I are a team and we decide on how things will be in our home when they’re here. He won’t tell her to stop calling and texting for non-child related things all the time because he doesn’t want her to get pissed at him. And he also won’t stop going to fix things at her place, that the kids break, because her boyfriend apparently can’t fix anything or he doesn’t do it the way my husband does it. No, he isn’t having an affair with his ex, so that isn’t the issue. The issue is her overpowering need to tell my husband what to do or he won’t see his kids and also him feeling that he’s gotta cave to her so he can see his kids.

What can I do?