It's a complex question. The answer depends on your goals, the circumstances, and the probability of actually achieving anything (and if so, what?)
It's possible that a tough customer like your mother won't even consider the message of your letter, which will presumably be heartfelt, and may just dump it in the trash, perhaps un-read. So you can’t actually MAKE her accountable. She has to be willing to be. How would you feel if that happened? Is the point to do something, anything besides silence? If so, ok. But with people like her, often there's no satisfaction from them.
I don't know who the other players are in this. Do you have siblings? If so, do they trust and believe in you? If they do, you might want to discuss this with them, and if it works for them, write an “open letter” to mother and them both. When people do this in families, it can clear the air by making the complaint public. But don't do it if you may be seen as the bad guy. Often in dysfunctional families, if one person speaks the truth, they are scapegoated. So find someone to strategize with.
I don’t know if you’ve ever sat down with a counselor to discuss this situation, but I know that if it were me, I certainly would. Make it a woman. And make her VERY understanding and accepting, the reverse of what you’ve known from mother.
I don’t think your mother has earned the right to be “left alone in her last years” if she’s treated you as you say. Unless she’s senile or out of touch with reality.
Some people are just “impossible.” They’re vicious, or so lacking in empathy that trying to get emotional sustenance from them is like trying to get nourished by drinking our own bodily productions. We just gag. In those cases, we just have to grieve the fact, find others to give us what we need, and go on with our lives.
So if your attempt to reach her might be useful, either successful in getting through to her, or useful in the family to make the point about what happened, ok. But don’t poison yourself!
I couldn't help but notice your post. It caught my eye. It sounds like you and I are very much in the same boat-so-speak. My mother and my siblings have abused me almost to the point of death. I now have a rare disorder where I have life threatening seizures. The trauma and abuse is so bad it is beyond human. I have been the families scapegoat my whole life and things have just recently come to blows. I gave up feeling guilty about standing up to my mother even in her elderly years. She is 79 years old. She has gone to great lengths to destroy me. She is so self-centered and self-absorbed that she is willing to sell out her own daughter. She has succeeded in turning my who family of 12 members against me. She is even going so far as to tell releatives how horrible and mean I am to her all because I stand up to her. Boy, no one stands up to my mother and lives to tell about it. I have been threatened to be killed on more than one occasion. I will no longer take her ****. Mother or not. I am a human being and she is not going to change that. someday I will write a book and I will have the last say. Hang in there. dont't let her short change you. Your a good person and don't let any one tell you otherwise. sammie