Dear Purplesofa
I am sorry to hear of your and your mother's loss. I'm sorry that I don't have the letter you are looking for, however maybe you can help your mom to appreciate that if you had a child that died she would not expect you to live up to any commitment that you made prior to that childs death. And our heavenly father is far more compassionate than we could ever be. So the fact that she gave her word in no way obligates her in God's eyes. I will be praying for you both.
RV