I totally understand where you're coming from my friend. Hope things go well.
With regards to the delivery, I like the idea of a letter as she is likely to keep it and read it over and over again. Not that she would forget the words but I know if I was a WAS the letter would hit me more than the spoken word. There's a certain formality about it that makes it kind of official. JMO.
Besides, in my case she dumped me through email so... . (and I did read it over and over again)
Freshman Class of 2012
M-49 W-42 1D-10 T 10 YEARS M 9 YEARS EA/MLC 07/2010 Separation 28/05/2012 PA confirmed 31/07/12 W Asked for D 31/07/12 D on and off the table since then