I'm sorry you're going through this. I have done the letter exercise a bunch, including two or three letters to my mom. Typically, my counselor asks me to read any letter I write out loud. It's never easy, but it always helps and is typically insightful. Once, as part of a therapy anger exercise on a trauma healing retreat I went on, I wrote the letter, read it out loud to everyone in the room, and then threw foam bricks at a wall while yelling anything I felt the need to. Yelling out loud isn't really my thing, but throwing the bricks after reading the letter and ripping it up was a very helpful component. I know some people will punch a pillow.
We're here for you as you work through this.