I agree with everyone who said don't send the letter. I've been there/done that. It only pushes them further away.
robx made a good point about standing up for yourself.
Think assertive. It's firm in a neutral POV. Take the emotions out and stand up for yourself. No attack or defense.
As robx points out, the balance of power totally shifts to them. My W took that power and actually wielded it like a sword...lol. I think back at how freaking spineless I was and this is again another mistake we all make in the name of 'not rocking the boat'. But pick your battles wisely.
They all run a script. And reading your back story here and things like, "she's only thinking about herself", can't she see this marriage is repairable (no she can't. Somehow they've gotten hold of a crystal ball and can predict the future), etc..This all the same behaviors they all show. Think of her as a robot who is running a program, then remember she can't help doing what she's doing, jump off the roller coaster and watch her go up and down.
DETACH
MySitch Me-47 STBXW-41 D-5 S-8 ILYBNILWY-01/08 Want a D- 01/09 Physical Sep-01/10 D filed-06/10 Got 50% custody=09/11 Ride that wave!