I know your DD doesn't seem to want to do anything but hurt you, but she darent do that to her abusive f, and he's the one hurting her. She will feel extremely conflicted about her situation and wants and needs you to put boundaries in to make her feel safe.
In the same home as him you can't possibly do this, and shouldn't try as you make things more dangerous for you all.
If you have already decided to leave, speak to WA, ask them to help you with steps to do this safely for all of you.
Do not go behind his back or confront him until you can do this safely. You are already to blame for everything, and like you said, it won't stop, he might promise but nothing changes, and it is only at times of you leaving that hea even prepared to acknowledge that he's a problem but he really is the issue.
He should be keen to see his DC get the help they desperately need hes an appalling f, and when you leave take them with you. She will suffer further if you leave her there.
It's not that you're not a good mother, its that he will not allow you to mother, but you can once you are out.
Better still will it be possible to get him out with an occupation order?
Also to Penguin 