I'd much rather be addicted to diet soda than alcohol. At least you're not going to get a DUI for drinking too much soda. We fret sometimes, it's part of the recovery process. I'm sure someone, somewhere would have a problem with it. But those are the people who's advice I tend to take with a grain of salt.
If diet soda is your biggest problem, I'd say you're doing just fine. Keep up the good work!