I also have a bad Diet Coke habit. Drink cans and cans a day. I hear Elon Musk and Bill Gates also drink copious amounts. I wonder what it is about Diet Coke that's so addictive.
(I wouldn't be surprised if research eventually concludes regular Coke is safer than Diet Coke, even given the problems with HFCS. So, if trying to quite Diet Coke, you might 1st want to try substituting regular Coke... which you might find more satiating and thus consume less of, by volume.)
Caffeine on the other hand has a well understood and documented mechanism of physical dependence, but its considered extremely mild compared to substances traditionally considered addictive (nicotine, alcohol)...
(I suppose we could raise the same question about food in general. Obviously food is necessary, but can it ever make sense to talk about a 'food addiction'? Or perhaps more accurately, an 'overeating addiction'? I think it can, in some cases. For someone wondering why they feel 'addicted' to diet soda, the high levels of mood-altering phenylalanine from the aspartame is highly likely to be a factor.)
What Diet Coke makes me crave is starches - "crunchy salt" in my wife's parlance. Potato chips, popcorn, crackers, stuff like that.