I cannot believe you find diet coke tasty. I would highly suggest you try to switch to tasty and time-tested sources of caffeine such as coffee and tea.

Of course, taste is a subjective experience but when something is so OFF (diet coke is not even a food and not a drink, in essence - it is like a solution of a caffeine tablet with harmful substances added in), you need to try other options and see that perhaps you find or discover something that you like. Also, traditional sources of caffeine offer a myriad health benefits and diet coke just offers caffeine plus is harmful due to the additives in it.

Try various flavors of tea etc. - maybe you will happen on something palatable. Hot tea, cold tea, green tea, black tea, white tea, ... flavored tea...

I do not think that it would be helpful that much to think of this as a habit to kick. It would be more helpful and you would feel more positive if you conceive of it as a trial of better, tastier, and let us say more "refined" if you excuse it , choices until you like something suitable that will replace the diet Coke.

It is also good to learn to pair food and drink in ways that benefit both in one meal, and diet Coke is pretty much a killer in this department, as far as I know.