I really need some advice.

Some people just never learn! It is the most disheartening feeling to feel like you don’t know who or how to trust. I feel at this point It’s not as rewarding as one might think to be so empathetic towards others feelings when your own feelings are disregarded by those same people. My BD has been in my life for 6 years and we have twin daughters together. We had split up due to his alcohol and gambling addiction as well as verbal abuse, cheating and on occasion physical violence. Immediately after I left (last January) with the kids he picked someone up at a bar. That someone had regularly spent time around my kids without my knowledge or my knowledge of who she was or even what her name was. It was months before I finally found out from my kids who she was. Things were off to a rocky start because he had already done his demonizing of me and the playing victim...he turned her against me before I even knew her name. At this point I had felt so disrespected because I was constantly being lied to “to protect his new relationship” and my children were well involved before I even knew her name that I figured my next step would be child support and establishing a parenting plan. He never helped financially since the split so I felt it was necessary to force it if need be. They broke up and he came to me wanting to get back together and move back in together...which to my stupidity I actually considered and believed he was being true about his feelings...that is until she answered his phone one night when I called. So, I filed papers in July and they were finalized in July. Basic every other weekend the dad gets them kinda plan. However, I let him have them every weekend and pretty well anytime he asks or else I’m accused of keeping the kids from him and being selfish. She and I started communicating at that point after realizing he’s playing her against me and me against her...he’s playing both sides. I warned her about what I had been through, she told me about what she’d been through. It got to the point where I was always put in the middle with the “he said she said” drama between the two of them and I literally felt like I was vicariously living through her all the reasons I left him for in the first place. My relationship with him has been total shit since she’s been involved...we can’t even hang around each other or go do things with the kids without it being disrespectful to her. He tells her one thing and tells me another...then she and I talk and it blows up in his face...honestly She and I had become friends confiding truth in each other because neither of us could trust him playing us back and forth. I went as far as involving her with my kids and their schooling as well...I made a full effort to accept them together and her being a part of my kids lives. This last time they split up he had convinced me how fake she was, that she’s disrespectful about me etc. I have been so beyond mentally exhausted from the back and forth that I am to the point of saying screw everybody’s feelings. I know I have no control over her being around my kids and that part is hard because I strongly feel my kids should not be involved. He twists things to benefit him even if it has the opposite effect on me or her. For example, I invited him and the kids to church yesterday....and he told her that he was being forced to go or I wouldn’t let him have the kids...all while telling me how he really wanted to go. He told her that so he could go and in her eyes make me the bad guy. She had mentioned to me last week about taking the kids to the pumpkin patch and I let her know that I already had those plans and it’s a tradition for me...behind my back they took my kids yesterday against my wishes and she even had the audacity to post the photos on social media right in front of my face. I honestly feel like overreacting. Something definitely has to happen..something needs to change. If anyone has advice please share it with me. I feel like it’s all getting to be overwhelming and disrespectful.