In my personal experience I keep having the same problem of cutting diet coke out of my diet as it makes my knees sore, and I always seem to come back to it and start drinking 3+ litres a day.  A never ending cycle.  I've had 1.5Litres since lunch.  This site is a timely reminder to quit again, and try to make it stick.  In fact I've just worked out that my clinical depression that I started getting about 3 years ago coincides with me starting to drink large quantitys of diet coke!