Officially ended relationship in May. One DD. Had NC for nearly four months (his choice - no interest in seeing DD) - which was devastating.But near the end of the four months, although I was still heartbroken and low, I was beginning to reach a point of acceptance where I no longer expected him to come back/apologise/be a better father.
Then he got back in contact, made lots of promises which I stupidly believed. I completely fell for it all, and started seeing him again. Then I found out that from a couple of months before we split there had been an OW who he was still in a relationship when he got back in contact with me.
I had my suspicions there was an OW when he first went NC with me and DD after we split, but it’s so much worse knowing the details.. she didn’t know about DD, she saw old messages on his phone between me and him and confronted him and he claimed that he hardly knew me and we had no children together.
He went NC again with us when I found out about the relationship, but we have a mutual friend who is still in contact with him and who keeps suggesting that I contact him, go to a certain place at a certain time, and that I should forgive him and how he is planning to visit DD/get back in contact. I cannot imagine why my friend would say these things unless my ex was feeding them and asking for them to pass it along to me.
This current NC episode from him has lasted for a few weeks now with him only making contact once and briefly seeing DD.
I feel completely trapped knowing that he’s probably going to get back in contact sooner or later and refuse to be mature and keep our conversation solely to DD. When we were together if I ever tried to break up with him when he treated me badly he would accuse me of sabotaging his relationship with DD and that she would hate me for ‘sending him away’ despite the fact I have NEVER stopped him seeing her, I’ve tried all I can to encourage contact but it’s just been a total waste of time. She doesn’t even recognise him anymore but I feel like he wants to keep me on the back burner for occasional sex or in case things don’t work out with his current girlfriend.
Not sure what I’m supposed to do really, I hadn’t even recovered from him fucking off the first time and now I have to deal with the lack of closure and knowing this will drag on for the foreseeable future. Having a life long tie to someone so awful is really bringing me down and making me feel quite depressed 
. Any advise appreciated. 