Just send her a short letter, assuring her how fine and happy you are, that you love her unconditionally no matter the different religious views and feelings you both have.
I also got letters like that. Usually I write something like that:
"I have a close relationship to my heavenly father and I am happy to be able to have faith and live according to my conscience. I can't follow any longer the Watchtowers rules (for example shunning, blood, child abuse), as they are going directly against my conscience.
I wish that you find a lot of joy, love and satisfaction in your personal faith and relationship to god. Be assured that I love and respect you unconditionally."
I am sure this is much better than to write a long controversial letter.
You can't change the faith of your mother, only she can when she is ready.