I'm frakkin' confused. She's moving forward with conviction but still sad about the ending of this. And she even thinks/thought that C WOULD allow us to reconcile - but that there'd be problems in the future.
So, WTF?
As much as it sucks, you are in a marathon if you want to reconcile with W. You must consider that reconciling will take months, possibly even years. Patience isn't a virtue here -- it's an absolute necessity! So, WTF can you do? You can keep working on your changes. Have you determined all of the behaviors you exhibited that contributed to the breakdown of the M? If so, have you been working hard on changing those behaviors? Finally, you have to understand that W will not believe those changes are genuine right now -- it will take at least 10 weeks (according to DR) for her to even consider that your changes are sticking and will remain consistent for the long run (and honestly, it will probably take longer than that). If you can keep the changes going and make them permanent, and show her that you are a new and improved person even AFTER the D, she will definitely begin to believe you have changed. Maybe then (if not sooner) she will begin to consider giving you another chance.
Also, please remember that you can't verbalize that you're changing, that you've changed, etc. You have to simply SHOW HER whenever you're around her (gaming nights will work well for this). You can't verbalize it because then it seems that your motivations to change are ONLY to get her back (insinuating that you wouldn't/won't make the changes for you because it's better for you). The changes seem artificial when you do it that way. Another way to SHOW her your changes without her being around is to SHOW them around your mutual friends. Be that new and improved guy around those who will also be in contact with her at some point. If they see your changes, they may very well comment on them to W.
Doing these things -- being ultra patient, stopping the chase, making your changes genuine and permanent over a long period of time -- will show W that you A) respect her needs, and B) have had an epiphany about yourself and who you want to be. They will plant seeds of doubt in W's head that tell her, "Maybe there won't be problems in the future if we reconciled. Maybe he can change, or already has. Maybe...". Does this make sense?
It's difficult to be patient right now because this is all so fresh for you, but you really must consider the benefit of it. Focus on you, your changes, and giving W the space she needs (and not doing things out of spite regarding the D that will come back to nip you) -- this will give you the best chance at reconciling (pre or post D), and will also make you a better, stronger, healthier person.
Oh, and please don't mail that letter. It was healthy for you to write it though -- just file it away. Journaling feelings without sending them is sometimes the best medicine.