I gave up my afternoon diet coke a few weeks ago and actually don't miss it. It was giving me major sugar cravings in the afternoon, and now I don't have them. (incidentally, I also tried to cut out other unnatural sugars, so this was part of that bigger picture change).
I mostly just like the carbonation, so instead I bring a 1L bottle of seltzer from our soda stream. It fits the bill perfectly.