Good luck! Soda is still my one last vice. I need to give it up (AGAIN), but right now I'm giving up alcohol, too, and I can only do so much at one time!
Really wierd, for me once I stop I can't have ANY. I gave it up a couple of times last year, once for almost 3 months, but as soon as I start with one, it goes downhill.
Unfortunatley there are no 12 step programs for diet soda.
You can do it though! What kind of other drinks are you drinking (besides water?) I liked to have tea, but also some flavored mineral water, or plain mineral water flavored with stevia ... just in case I had that craving for something fizzy and flavored.