How can a 0 calorie drink make you gain weight? It doesn't even have that much sodium in it.
But -- it sounds like you're a little overboard on the Diet Coke.

Water is best for your body, though there's certainly room for the occasional diet drink. I had two cans of diet soda a day during the year I was losing weight and can't say it stalled me.
How about slowly adding in some water? I find it easiest to drink water with meals, so maybe switch to water at lunch, get into that habit, then switch at dinner etc. Until Diet Coke is a treat and not your staple drink.
Believe it or not, your body will learn to love and crave water. It's truly refreshing! Try keeping a pitcher in the frig with sliced lemons or oranges in it. It's such a clean flavor!
Don't worry, we've had diet soda addicts here before who successfully transitioned to mostly water. Just do it a little at a time!!
Good luck!
