I’m similarly conflicted about my DM. I’ve accepted that she didn’t know that my F was sexually abusing my DSis and me. (My DSis is less prepared to give her the benefit of the doubt.) But the reason she didn’t know is that she simply wasn’t there for us when we were growing up. She was distant from us emotionally, probably due to no fault of her own, but because she herself was badly damaged by having been orphaned at 10 and sexually abused by the uncle who became her guardian.
My F died 23 years ago, so it really is just about my relationship with my DM, and her relationship with my DDs (12 and 9). It isn’t just about the past.
There’s also my DB, who also suffered sexual abuse as a child (though not by my F), but he was groomed into joining in with abusing us. He claims no memory of this, and he is badly damaged as an adult. My DM has always guilt tripped me by telling me how lonely he is, but I’ve kept him away from my DDs and very much at arm’s length myself for the sake of my own mental health, which isn’t that strong. I have PTSD and I’m on anti-depressants, as well as having Long Covid now.
She’s very good at manipulating us so that she’s the one who has reason to be upset. I know that it was a massive shock to her when we told her about the CSA (again assuming that she’s telling the truth), but she has this way of bursting into tears whenever the subject comes up and asks us ‘not to ruin her time with her DGDs’ by talking about it.
She’s also very controlling in everyday life, particularly around my DDs. She used to attempt to take over the parenting role, but I have because more assertive in recent times, so this doesn’t happen so much.
I’m realising that, at 82 now, she isn’t likely to change much. So I’ve reduced contact with her, both because of the past, as it’s triggering being around her sometimes, but also because of the way she is generally.
Your anger is understandable, OP as your Mum should have put you first, although obviously she was in a difficult place herself. But it doesn’t sound as if she’s even acknowledging the role she played in failing to protect you when you were a child.