I understand the need to write a letter. I wrote one and gave it to my husband.
It did not make things better, but I am still glad I sent it.
However, if you send it, do not, repeat Do Not, ask anything of her. Not that she thinks about what you wrote, and definitely not that she puts the D on hold and goes on a walk with you. That is 100 percent pursuit.
Me: 44 H: 44 Kids: 20, 16, 16, and 10 Together/Married: 22 years H announced he was emotionally detached and considering D: 4/4/16 H announced he is going to try to stay and reconnect: 5/1/16