Don't send that letter. Write it, because it will be cathartic to get out some of what you're feeling, but if you send it to her, you have to assume it goes straight into the divorce court record.
Me: 46 W: 44 Married: 17 Together 21 D13; S10 BD: 03.03.15 (Not attracted to you) Almost 2 years trying, alone, to save marriage Status now: Divorced (effective 06.13.17)