Definitely good to get it out. A good therapeutic process. I've written dozens of these in the past, seems like ages ago. But never sent them, of course. Sounds like you are not planning to either, which is good.
Just in case you need any reinforcement about not sending, this might help -
Why not to write a Blame Letter:
https://bpdfamily.com/message_board/index.php?topic=123984.msg1221875#msg1221875Silence is golden. Good luck in your processing!