Wow, that is a lot of diet soda. Like, medical researchers and nutritionists might be willing to pay you to monitor how this affects your body long-term "a lot". Also, that sounds more like 5 liters per day.
What else are you getting from McDonalds every day? What's your general health/obesity level like?
Do you think you have ADHD? I've frequently taken in massive doses of caffeine as self-medication for that, and it's pretty common.