You can't take back anything you write in the letter. It's not even like a heat of the moment statement that you can retract. A letter shows a lot of deep thought and effort which she may always remember.
A few questions for you: 1. WHAT do you deep down hope you will accomplish by sending her this letter? 2. We know what you want to say, but WHY do you want to say it, and WHY now?
She MAY never come around, but what if she wanted to? She might be doing the most unconscionable things right NOW, but dismissing her friendship with words you say now might make her feel like she could never come back to you, even if you both wanted to, or realized you had to, start as friends first. Also, just like with words and ultimatums, you'd have to back up your written statements over the long haul for her to believe in you regarding what you say (and write). Do you really want to be on the record of making this extreme statement?
Let your actions show her what you want to write. Meaning, if you feel you can't be friends with her while she's doing this, then keep staying dark and detaching. If you keep at the detaching, that will prove more effective than any words you could write. I think by writing, you're just showing you're losing your patience with the detaching process and essentially going back to pursuing her. And again its an extreme statement that you might wish you hadn't made a while from now.
I hope you don't send it. Think long and hard and with some futuristic thoughts before you do.
M-34 XW-32 D-7 Found OM's presence 4/09 Separated 12/09 Divorced 8/10 GREAT relationship as coparents since 8/10