I used to drink diet soda a lot. However, it did stall my weight loss. In my experience anyway because it made me retain water. I gave it up completely. Now, I drink a diet soda once a week or every two weeks.
Like any bad habit to break, it takes time, and you need to be ready. I have a friend that has tried every diet from weight watchers, to quick weight loss, to nutrisystem, etc, and they all told her to stop drinking diet coke. She only has one a day. (it's more like her morning cup of coffee) and she hasn't been able to lose any weight.
Sorry if I wasn't much help! Just wanted to share my thoughts on it.