This should help narrow down what is causing your lack of energy. The diet drinks have nutra sweet, and this has been shown to promote hunger. So many report feeling hungry within an hour of drinking a soft drink. Also, if you intake a processed food or drink, it robs your body of B vitamins and can even block absorption of calcium into the bones. There is also a lot of research showing caffeine or too much of it can promote cell growth.
Water is the best way to go.
If you must have a soft drink limit to one per day, but drink water with it which will help flush the caffeine out of your system. Sorry if I burst your bubble on soft drinks. It takes 6 hours for the body to get the water out of the soft drink. Many swear by taking Bee Pollen as a supplement for energy. Honey is just the byproduct.