First of all...your ex sounds like a crazy person. So sorry you are going through this. Focus on S and keep the door open with your D. Everything you do and/or say will get back to your D (and our ex unfortunately) so be careful. Don’t bad mouth your ex in his presence. Don’t make him a go between with your daughter. That will put too much pressure on him and he will feel like he has to fix things which will blow back on him when he is with mom.
RE: your question to me. How do you bring them together? The answer is... YOU don’t. That is up to your gf. There are lots of books and YouTube videos on stepparenting. Since she is resting most of the time anyway, maybe she should start educating herself. It will help with parenting her own child as well.
I became my SD21’s stepmother when she was seven. Her mom was an extremely controlling emotionally unstable person...still is. So SD’s loyalties were REALLY tested. I did not try to force anything with her. I was just a consistent presence in the home... welcoming (made a point of greeting her and saying how happy I was that she was there even if I wasn’t feeling that way), kind and calm and when she was struggling with loyalty issues, I didn’t make it about me. It wasn’t easy. There were a lot of ups and downs and I was the main person dealing with her mom. I had to work extra hard to not let my challenges with her mom bleed into my relationship with her. To be honest... there were some days I didn’t want her in the home because of her mom and the stress she caused. SD never knew that and she never will.
Luckily, I was not a parent when she first came into my life so we did get some concentrated time together with either just her and me or the three of us. Be forewarned though...when your gf has her baby, she is going to experience the kind of love that occurs when you bring a life into this world. I love my SD21 a lot...but it pales in comparison to the love I have for my own kids. I was almost 40 when they were born so mature enough not to let that show. I’m not sure your gf has the maturity to do the same but I will cross my fingers for you. As an aside, because of my role as stepparent, when SD21 was in full rebellion phase, I handled it much better than her mom did (because her actions didn’t feel like a personal rejection which often happens with your own kids) so I became a confidante (sometimes she told me TOO much) and someone she could turn to for advice when she needed it. That relationship endures to this day even though her dad and I have divorced.
Anyway...hang in there. This isn’t going to last forever. Focus on what you can control. You can do this. (((HUGS)))
