(((di))) I am so sorry for your pain. I can only imagine how difficult this all is for you. I just wonder what you were hoping to accomplish by sending your H the letter. Do you expect a reply? If he did reply, what would you expect him to say? Do you want a confession?
All over the email you keep saying things about you and your feelings. What your H is going through isn't really about you...it is about him. He is unhappy and he believes it is the M that is making him unhappy. You need to give him time and space to figure out what the real source of his discontent.
Sure he sees your history differently than you do...he has to justify what he is doing...if he let himself see that the two of you have had even a relatively happy life together, then he would have no reason to do what he is doing. The more you push for answers, the more emails you send, the more you try to get him to see your point of view, (in his mind) the more justification he has for his actions. He has to come up with his own answers in his own time and he will see your point of view only when he is ready to.
I hate to sound like a broken record but you need to let go and realize that even though your world as you know it is falling apart, this isn't about you. Show your H your strength. Show your H that you may not like his choices but you accept them. Show your H that you can grow and thrive despite what he does. Be the lighthouse.
Hang in there di. I believe the sooner you drop the rope, the sooner things will get easier for you.