You're not the complete selfish bitch, your mother is. She denied his abuse of you, she failed to protect you, and in fact enabled him to abuse you by staying with him, because she put herself and her own needs first. You were way down on her list, not number one as you should have been. That makes her a first class shitbag by any decent standards.
Sorry for being so blunt, and my guess is my comments will make you feel defensive of her and upset that I'm slagging her off so much, but what she did was vile, and your compassion - your obvious compassion, because unlike her, you sound like a lovely, truly caring person - should be reserved for you yourself, and not squandered on her.
You sound like you still doubt that you are entitled to prioritise your own needs, and that you think you would be a bad person for not allowing her to trample all over you to make herself feel better, as she did in the past. But you wouldn't be. There is nothing bad about protecting yourself, caring about yourself; in fact for those of us who were let down and abused by our parents, caring about ourselves is an act of courage and should be something we aspire to, not feel guilty about.
Your DCs need you. They deserve you to prioritise them, and yourself, above her, every single time. You owe her nothing. You didn't bring her into this world, you didn't make any of the choices that led to her being where she is now, you are not responsible for her; and if she stil tries to make you feel you are, that is just another way she is still emotionally abusing you.
Sorry, I have no direct advice on how to deal with your SF's passing, but as far as dealing with your mother's inexcusable guilt tripping of you in the aftermath goes, please try and remember you are not the bad guy in this scenario. Remember, the reason your relationship with her is "strained" is because she failed you massively, not because of any failings in you. This is not something you can put right, because you have done nothing wrong.
I suspect that underneath your numbness there is still a good deal of anger at both of them, and with good reason, but I can understand that you might find it hard to make headspace for that, given how full on your life is anyway. But whatever you feel, it's perfectly valid, and it's your perfect right to handle all this in whatever way is best for you, not for anyone else.
Wishing you a peaceful start to the new year, and a peaceful continuation, come to that!
(Sorry such an essay,
may be involved!)