OK that is totally fair and honest of you, and I would agree that if there is too much hostility the letter is not going to be useful.
Maybe stick around, posting and reading, for a couple more weeks, until you can be a bit calmer about things and then try again?
However, writing the hostile and angry letter, but not giving it to her, can maybe help YOU a bit by getting your most honest emotions out there...outside of yourself, even just by a letter she will never see. That's just a thought, and these letters can be a useful therapy tool even when they don't go to the intended reader.
Have you thought more about trying to understand she is not doing this to tick you off on purpose like I said yesterday? I know this will not ease your long-running anger in just one day, but does the idea of it help at all?
It is ok to say "no it doesn't help me, I still feel angry". I can accept that. I was just doing a bit of follow-up.
