My vote is DO NOT send the letter! You are absolutely right, it is just angry "venting" so no good will come as a result of it. The reality of the situation is that after reading your letter, he will not magically "see the error of his ways" The only thing that will happen is that you will only come off looking like a b!tch, thus making him feel more validated in his decision to leave you.
That being said, I'm so sorry you have to go through this. None of this is fair to you, your history together, or your children. The only thing you can do is what you have been doing which is to carry yourself with dignity, pursue activities that make you feel good, detach yourself from his immature craziness, and protect yourself and your family throughout all of the stages of the divorce.
The constant crying sucks, I know. If you need to cry, do it. I think Michele suggests breaking some old plates or something? Regardless, let it out - but not in front of your H/XH. Right now, he doesn't want to see/hear any of it.
In my sitch, I got to a point where I was tired of leaning on my friends and family (felt like a constantly sobbing burden) and decided to go to a therpist to vent out all of my feelings...including all of the stuff that was totally unreasonable/nutty. Was one of the best things I ever did. Probably the easiest $ the therapist ever made because I paid her to sit and listen to me cry/yell about how $hitty things were and how unfair they were. After a while, the crying subsided and I started to feel "like me" again.
My XH did some odd stuff during the divorce too. Although he filed, he did nothing to advance the divorce, which was maddening. In fact, he was hounding my attorney with crazy "deals" that no person in their right mind would agree to. Lots of bizarre delaying - but not to reconcile or talk about relationship. Don't "engage" in the nuttiness.
Hang it there. You will get through this.
Me: 30 H: 31 Together: 8+ years, Married: 4+ years Bomb: May 2007 Divorce final Oct. 2008 No kids, 1 super-cute yellow lab