I've been through this, but with my abusive F. Looking at it now, he was a definite narc and subjected my DM to years of EA and paranoia. (He constantly accused her of being unfaithful to him.) He was tyrannical in our earlier years, bullied her into us all going to Saudi Arabia for a year, when she was a young mum with 3 DC of 5 and under. Then he insisted on getting a dog when she had made it clear that she didn't want one. (She's since told me that she cried about this whilst he was getting this dog.) Naturally, she was the one who looked after the said dog.
He physically abused my siblings and me, he smacked us really hard regularly. (DSis and I suffered SA from an early age, too.)
Despite this, he was able to play the martyr because of his wartime experiences. (He was Czech and had lived under occupation.) Then latterly because he had Parkinson's Disease. I understand now that he was simply a narc who happened to have Parkinson's Disease.
I remember watching him when he was dying in hospital, back in 1998, and willing him to die. It screwed me up afterwards, and I really grieved for him, but now I think it was as much guilt as anything else.
Now I'm grateful that he's dead, I have been since the scales cams off my eyes and the memories which had been repressed for years came back.
My relationship with my DM is also tricky, as she is controlling, but I've learnt how to stay fairly low contact with her, and she's not too bad in small doses. I also know now that she was a victim too.
So I would be very wary about responding to this letter, OP. Your mum hasn't actually accepted any responsibility for your actions, she's put the blame on both sides, which suggests this is just a way of reeling you back in.
I would suggest sharing this on the Stately Homes thread on the Relationships board.