Most Helpful Guy
Sorry about your mother. Even as you support her, surround yourself with people who will help support you during this heartbreaking time. This guys is clearly not one of them.
Regarding the letter, I've found writing helps me explore and organize my thoughts, and the process of externalizing them makes me feel better. If that sounds like something that might help you, then go for it. Then fold up the letter and stick it in the bottom of a box somewhere. That's right, don't send it. You may even find after you've written it that you no longer want to, but either way, it's better not to pick at scabs, so to speak. "Closure" is ultimately something we arrive at in our own minds; the other person rarely cares, and that can make one's frustrations worse.