Firstly, I am really sorry to hear about your diagnosis

This seems a really complex situation that could take a while to "untangle" or so to speak.

I am glad that you are going to be seeing a therapist. When I saw one through some really rocky times, she helped me, bit by bit, begin to mend things in my life that I needed help with.

It's not going to happen overnight, but as the saying goes, if you chip away slowly at it piece by piece, it can get better.

Your mother is old and probably is saying a lot of things that she shouldn't be saying that are hurtful. I'm not sure it would be even worth you bringing things up with her. She could have some neurological things going on due to her age, I don't know. But I can see her as being the type of person who will get defensive and deny wrong doing. It's not that you're "letting" her get away with some of her unwarranted behaviour, it's just that you're going to rise above her hurtful comments, acknowledge them in your head, but don't speak them with your mouth to her if it's going to make the situation worse.