I think there's more than one issue here

Difficult -- One could be flippant and say that the project is complete (until you do drink some coffee!). If you're going to use GTD for something like this, you'll need to make the outcome of the project more specific (for example - don't drink coffee, but replace with herbal tea - or - don't drink any coffee until the end of the month).

Changing your diet - again, more focus on the outcome would help. But in this case, there's a positive outcome (as opposed to the 'don't drink coffee'). What do you want your new diet be like? How are you going to achieve this? Possible NAs might be 'make shopping list with new diet foods only' or 'look up low-calorie meals in cookbook' or so on.

I don't bother putting that level of action into my GTD system. I build those sorts of things into routine behaviours which 'always' happen. I happen to be 'not good' at mornings, so don't want to think more than possible. Hence I try to do the same things in the same order. In my case this happens to be 'get up, go down stairs, feed cats, make sandwiches for lunch, make breakfast, wait for wife to come down stairs, eat breakfast, clean teeth, put in contact lenses, get dressed, cycle to work' -- I typed that straight off!

I suppose it's that GTD doesn't do everything for you -- in fact getting GTD to work involves making your system much like my morning routine. Something you can do without thinking, so it's actually the easiest way to behave.

Not sure I've really answered you questions, but I hope it's useful.

Steve