so sorry you’re feeling lonely and disappointed - but that seems like a reasonable reaction to your situation. Honestly, you’re collateral damage in his war with BM.  Consider counseling for yourself - there is so much to unpack and digest.  Wish you the best.

I agree with counseling for yourself and eventually your SO. It definitely sounds like he is taking his frustrations out in you, and in turn it is damaging your relationship.

I had the same experience with my SO and it's an aweful situation to be in. Establishing boundaries and couples counseling did help us get through it. Eventually SO started going for himself which has really helped him learn how to deal with things better and has given him a place to vent his frustrations.

First and foremost, you need to adjust your attitude a bit toward your DH.

His kids aren't just picking up on their mom being mean. He doesn't sound overly respectful of you, and his kids are going to mimic his behavior towards you. If he hides all his good qualities towards you while they're around, and he lets out his frustration on you, they're going to learn that's what you deserve.

Their behavior can't be placed squarely on BM. Your DH, especially in your home, has a lot of authority to require his kids to behave differently. To teach them differently. To at least point out when they're being overly mean or hateful. His job as a parent is to correct that behavior, but he seems to slip into it in their presence.

Because you have children with him, I think couples counseling would be in order, then individual counseling. Counseling for him to work through his feelings about parenthood. Counseling for you to figure out why you accept this behavior, especially enough to have a second child with this man.

You also need to determine what you need out of this relationship to continue it, or at least to participate in it. Through therapy, you should be able to share that with your DH so he knows what the goal posts are. Take time to think this through so that you don't set goal posts that you have to change later, or change repeatedly.

As for BM, your DH needs to follow their court order to the letter going forward. Pay what it says, take visitation as it says, and communicate as it says. If BM doesn't like that, then she can hire an attorney on her own dime to request changes to the CO. BM is high conflict, and that means dealing with her differently than if she weren't. She gets the court-ordered treatment and nothing else. If the kids start in with DH about him being a bad dad, he can correct any lies, tell them the CO is none of their concern and he's sorry that BM dragged them into adult matters, and then message BM on the app or through the approved communication (in writing) to stop involving the kids. Document and only get the attorney involved if it escalates.

You have to come to terms with the fact that BM can drag DH back to court as often as she can afford. Your DH doesn't always have to go with an attorney, unless it's something he actually cares about. CS won't change unless circumstances drastically change (or your state is very BM-friendly). Custody won't change unless something drastically changes. Your DH needs to identify what's important, and a lot of this is game playing. BM doesn't actually want more money or more custody. She just wants to stick it to DH. If he stops playing the games, then she'll likely stop playing, too, because it's not fun anymore.

Overall, your have a DH problem. He needs a spine against BM, and he needs to reflect on his own behavior and what it teaches his daughters. You need to stop putting up with him treating you like crap. Know what you want, tell him that, and call him on his crap when he's behaving like an ass. Don't just try to act sweeter and sweeter because that just ends up looking like permission. Tell him he's being a jerk, that you won't tolerate it, and if that's how he's going to behave, he can go elsewhere for a while because you won't be his emotional punching bag and disrespected just because he's upset. HE is responsible for his emotions. HE is responsible for managing them. If HE can't balance being a good parent and spouse, then HE needs to get help to figure it out, whether that be therapy, self-help books, his own forum, or a combo.