Wrote it and then posted the fucker.
Got a nasty letter back - I was 3 months pg at the time! It showed me, once and for all, my mother's true colours, and let me keep them for posterity. She took the piss out of my anorexia when I was a teen, my former drug addiction and denied all knowledge of the activities of her paedo husband. Which I know she knew all about cos I told her. 
Anyhow it blew the lid of the idea that somehow my mother is a poor misunderstood lady and I am, somehow, all to blame, and very unreasonable. Of course I felt guilty (my poor, poor mother!!!) but I got over it. I had lots of psychotherapy and counselling. I probably wrote a lot more letters too, over the years. But it was beneficial, eventually, to know that I told her exactly my side of everything, what I felt and thought. And to know that she didn't really give a damn.
Never seen her again, my choice.
Amazingly, apparently she hasn't cut me out of the will, although she is welcome to, and I fully expected it. We'll see.