I don't know how to let go other than completely blocking her from my life completely. But I do understand completely...my mother continues to deny the existence of her issues. She is perfect in every way and even when she does make a mistake, it wasn't as bad as it could have been...and so on.
I guess in my experience, its an all or nothing kind of thing...and yes I know that's not necessarily the correct always of things. But I'm either in contact with her or I'm not.
Also...something that helps me...just because I am trying to get Bette and manage my problems doesn't mean she (or anyone else in my life for that matter) is.
I wish you the best of luck and I hope you are able to detach. Lemme know how you did it...perhaps it'll work for mine too!