I have one can of Coke a day with my lunch - I can get away with half a can a day so far, but anything less I really tired and headachey and just plain feeling sick by 2pm. I don't like being dependent on caffeine =( but so far nothing's worked in putting it all behind me.
That sounds more like caffeine withdrawl than anything, look it up. Go without it for about 3 days and you will be fine again. :) Even a small amount of regular caffeine will change your physiology and brain chemistry, true coffee addicts can have terrible withdrawl symptoms while their body readjusts to a lack of caffeine. Try going for a walk at lunch instead of drinking caffeine - if you think you just need caffeine to stay awake, a little bit of getting up and moving around does the same thing as half a can of coke.
Also if you take extra strength tylenol or anything too, be sure and check the package. Some over the counter pain medicines have caffeine in them, because that can help boost the effects of the painkiller (for short periods of time).