NC, I'm so sorry for the tremendous pain she is causing you. I think the letter is a good idea. Keep them in a box and then later bury them, burn them, whatever makes you feel the best. Detachment is the key here. I know it's hard, but she is being extremely cruel. You have to protect yourself from her. Keep contact to a minimum.
Hugs
Life's challenges are not supposed to paralyze you, they're supposed to help you discover who you are. -- Bernice Johnson Reagon