How did you fail within two hours? Was there still some in your home or did you unconsciously walk to the store and purchase more? Getting it out of your house is key. It's important to raise the friction to act on the vice as much as possible or else you will engage in it without thinking.

If its caffeine you are physically addicted to, I will say the withdrawal is probably just as bad as a nicotine withdrawal. Nicotine withdrawal seemed to hit much faster (for me it was within 15 minutes of my last hit of nicotine vs coffee I just needed it bad in the mornings), but other than that the effects of withdrawal felt the same to me between the two stimulants. In both cases, if you are able to deal with a terrible migraine headache and feeling underslept and sick and nauseous for the two weeks it takes to get through these symptoms (in my experience at least), you will have successfully quit. I have cut back my coffee to maybe 3-5 cups a week (after a long initial detox) and I haven't used or craved nicotine in years.

Seems like an addiction you have through mental/memory association or routine that has been created over time for you. If "Diet Coke" is still bearable for you, or even if not, try switching to that to disconnect the addiction within your brain, then perhaps it may be easier to quit. I am not a psychiatrist though, just a suggestion.

Are you getting caffeine from other sources?

Try drinking tea, caffeinated or decaffeinated. I'm not sure why, but brewing tea has helped me stave off many such addictions.

Do other drinks / sources of caffeine work to replace it? I know it takes a couple of weeks to get over caffeine withdrawal, during which you may feel really tired or get headaches.

Is it Coke Zero specifically, how do you react towards other diet sodas?

The black label Coke Zero uses aspartame instead of saccharine, is strongly carbonated, and has a noticeable caffeine bite. Sorta similar to Barq's root beer, which is less flavorful than A&W and other competitors but has caffeine and was marketed with "Barq's has bite."

The newer version tastes a lot more like regular coke, but to get that flavor, it seems that it is meant to be poured over ice.