I could never slap you Eliza!
It's so hard... Detach and you end up with too much of what you wish for, ie. being completely outcast! And as it's "your choice" they won't even feel guilty.
Or, don't detach, and end up in a world of pain trying to have some form of control or effect on a situation involving people who would rather you didn't exist and don't give a shit about you. How much does your DH know about how you feel? Can you do a detach but with ground rules?
Our relationship was falling apart at the time and I don't think he gave a second thought to how it was making me feel. I didn't have the confidence in him to be honest about my feelings. So we drifted and drifted. We had counselling in the end and recently got very happily married. It was hard to put that treatment behind me but I knew he was a good man. He was just conditioned by years and years of emotional abuse from his ex and dsd was using all the same tactics she'd seen mum use. That and the guilt about initiating the divorce that he had never dealt with meant he handled it all spectacularly badly.
Luckily for me, Mums new partner is vile and so he is now the scapegoat for all dsds vitriol. She "likes" me now and as such things are a lot easier even though the person she is often disgusts me and I have to put that to one side most of the time.
Anyway, the reason I'm going on about all if that is that I took, and take, a lot of comfort in the fact that Dh's eyes have been open to his less than perfect baby and he is willing to confront her bad behaviour now. He is also willing to put me first when it's needed. Is there some way you can focus on the support he gives you and that you have him onside even if he still loves his children? I kind of see it that he loves me and his dd, but he really likes me. I know it's not a competition but these men are with us because they choose to be, their love for their kids is not a choice. They are slaves to it.
If our relationship was stronger back then, or if it happened again now, I would detach but set ground rules that if he was going to spend time alone with her it would be in the calendar with enough notice for me to plan something else, it wouldn't be extravagant, and we would keep our little rituals like Wednesday date night and Friday after work pub 
I would also want a full account of what had happened. That will sound like complete madness to anyone that hadn't gone through this, but the worst thing was when he returned after an entire day and evening out of the house and when I asked how it had gone and what they had done if just get a shrug or an "it was good, thanks". I felt so shut out and insignificant.