Your body doesn't recognize fake sugars as foods and doesn't know how to process them. In a lab they can prove "0" calories, but your body is different. These chemicals build up over time, there are also a lot of studies about diet soda a weight gain.
Can't give up the diet pop? Have a glass of water alongside it. Drink the water to quench your thirst and drink the soda for flavor. This will help you cut down.