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I've had to deal with similar feelings about my mother, also now in her 80s. One thing that helps (once I got past the initial slow-burn of anger) is not to have any expectations about her. She is who she is: self-absorbed, funny, intelligent, oblivious to my feelings. When I do put expectations around her, even simple ones, she cannot live up to them. Why then do I keep her in my life? Because she is my mom, I love her for the good in her and we can do fine for 2 hours at a time (about the length of our usual, few-times-a-year visits).
That said, I am very grateful to have known, albeit too briefly, what a real mom can be like. In many ways, my late stepmother was more of a mom to me than my biological mother.
I hope the LW finds peace for herself. It may take time, but it is indeed achievable.