I understand how you're feeling K....wanting to give her one last shot to rethink at this crucial point. While I agree with what Wonka said, I know you're going to do what you feel is best.
I wrote a letter to my H the week of BD (several months ago), stating how I now had tools for a good marriage that I didn't have before and I know that deep down he knows we could make our M work etc...it unfortunately did not have the affect I wanted and I wrote it before I read other books w/ knowledge on a better way to go about it.
If you end up following through w/ your feelings on writing a letter, the only thing I can thing of is writing a letter similar to the one exampled in James Dobson's 'Love must be Tough'.
I would say, start off the letter saying you still feel differently about ending the R, but let that be the only brief mention of R, after that point (IMO) you have to show you understand her POV ( e.g. "I wish there was an alternative where we could start a new marriage, but I understand leaving is the only way for you to be happy....") and then wish her well in life (in order for it not to be the pressure and all the things Wonka listed...instead attempt to "Lift in cage door" for her to feel free, but at the same time she got that one last gem of hope of R from you for choose to revisit or stick to her guns).
I wouldn't write a long letter detailing the whys and hows... such as the 3 page letter I wrote my H, which after reading he kindly suggested I leave his sister home and make a 3 hr drive back home alone. That was a fun day *sarcasm* lol
I know things must be tough right now, I can only imagine how you are feeling during this stage in the process. I hope you're able to do some activities this weekend to take your mind away from this for at least a short time.
me: 30 H:30 tgthr:7 m:4 no kids 5 counseling sessions initiated by H as a LR: Oct 2012 long distance marriage b/c of work since Nov 2012 official BD: July 2013 nothing filed 1/1/14 I dropped the rope