Husband wants a divorce because of his kids. How to stop him?

Right now, my husband is trying to leave me and its killing me. We've been married 3 years and have a 13 month old daughter. He moved out 7 months ago and has started the proceedings. The biggest problem in our relationship is his ex wife. They have two children, 20 year old boy and 7 year old girl. She cheated on him and left for another man, abandoning him and their kids. When that didn't work out, she tried crawling back to him and kept trying even after we got married. I didn't want that woman around us at all and I hated the fact that we had to take a portion of our income and give it to her, to take care of a child that she left. She insisted that we take the kids every weekend because she needed some "mommy time" but still wanted extra money. His son acted like a spoiled brat around me, but an angel around his dad. The girl would just flat out avoid me. I would tell him about he the children needed to respect me and he would attempt to fix things but those two still floated around acting as if I didn't exist. Things did get a little better when I was pregnant until about 9 months ago. It was our anniversary so we left the baby with my step-son. We come back and his mother is in my home, changing the baby. He said that he needed help with the baby and called her. He knows that I hate that woman and probably called her to spite me. Of all the people to call, he calls her? We argued and he said he's moving out and did a week later. Then my step-daughter, completely stop respecting me. I would tell her to do something and she would just walk out the room like I wasn't even there. I sent her "mother" an e-mail addressing on why she should live with her, and she forwarded it to my husband who then had the nerve to become angry at me. He said I was trying to get rid of her, which was untrue. I'm just tired of being treated like a doormat and ignored by a child. We argued for some months and then he moved out. We were going to counseling until 5 months ago. He said he needed time to "think about our marriage" and now he acts as if he hates me. I think his ex or kids are poisoning his mind because now he's against me. I went to a friend's party on Monday and he was there. I wasn't expecting him to show up, but I chose to leave him alone that night and didn't say a word to him. After some time, I left and I go out to my car to see him and this woman kissing. I confront him and he starts yelling at me. He says that he's on a date and I have no right to make a scene and I say we're still married and need to work us out. He then says that he hates me and that I'm a bad person for all the crap I put him and the kids through and he wants out. I'm not a bad person and I don't believe I've done anything to anyone. I love him so much and want this to work and for us to be a family. How can I make him see that?