Hi OP, I have been where you are and it was a long road for me to get to the point of cutting off contact. One thing I did manage first though was to stop having phone conversations, because she required it once a week and it would hang over me and make me so miserable.
I told her I didn't have time for phone chats because of work and DC, and it would have to be emails from then on. She kept trying to phone for years, leaving snidey messages, but I just didn't pick up. Partly it was true, I really didn't have time, but also why should i endure an hour of her nastiness just because she decided I had to?
I only ever saw her once every couple of months (she lives quite far away) but even that became unbearable with her constant demeaning comments, lack of boundaries and making everything about her, and raging and accusing me of all sorts if I dared to challenge any of it.
I finally said no more just a few months ago. The guilt isn't as bad as I expected. The truth is she has treated me like dirt for 40 years and while I knew that, I somehow fell for her version of events that we were close and got on. But it was all on her terms and if I ever stepped out of line it was awful. I just had to really see that I shouldn't feel guilty about hurting her feelings by saying no to having her in my life, when she has been hurting my feelings deliberately, for my whole life and not giving a crap.
It's difficult but if you can see it like this, it helps. I'm sorry you have a mum like this, I know how hard it is. 