Make an outline of the most important things you want to say...
This way if you have to refer to your "cheat sheet" you can just tell her that you didn't want to forget anything important...then just say it like you mean it...from the heart...she knows you well enough to know if your being sincere or not...
Also...I would leave less time between the talk and the D...I did mine 5 days prior...I think this almost pressured him to make a quit decision rather then think it over too much...the bad part was he went dark for 5 months...the good part is he did agree to jointly meet before a judge to stop the divorce and revert it to a legal seperation....
I think too you are putting too much emphasis on YOUR changes...I would mention how you are working on your issues and will continue to do what you need to improve yourself so that everyone benefits...pointing out this is something you feel you need to do for YOU...
Do an outline...state what is important...and let me read that...okay?...I find it hard to really focus on the whole letter to really comment much because I get lost in the "stuff"...now sure how your wife is but if it is too wordy I do get lost...to the point, quick, and sincere...that works for me...