What's a Mom to do?

I have been in and out of custody battles the last 2 years with my ex. The first time he tried to say I was unfit. The second was that my fiance was abusive and a drunk. Our lives have been turned upside down from the ex making accusations. My relationship with my fiance is on the rocks because of this. My children are suffering also. I asked the ex for consent to take the children to counseling because of the mean things that he has told them. E.G That he wasnt thier father anymore and that my fiance was after the first loss of custody to him. After the second time he is telling the kids that is my fault that the kids do not see him because he works all the time. Now he is drilling the kids for information again. I really feel bad for my children and they have to go through this. The school is behind me 100% and they know everything that is going on. He has made a promise to me that he will continue to take me back to court until I give up or he wins. This is not a game and he knows the only way to hurt me anymore is to take my children and ruin the family me and my fiance are trying to rebuild. This is not for my children but his own satifaction to ruin my life and not let me go on with it like he has. He has a new baby coming this month with his girlfriend and my children are suffering because of that to. He has told them that he is moving them up to the attic and the new baby will get the room that they now occupy when over at his home. What's a mom to do? I am trying everything I can not to lose faith but the court system is not helping by letting him get a way with all of his selfishness. Even though I have primary custody he is making it very difficult for me. His mother is always right there also. I really think that she is behind this because she wants my children. Can someone help?