Sweet Mother of All That is Holy. Almost a whole week has gone by of giving up diet coke, and no amount of excedrin, advil, or motrin will touch it. I have had to resort to drinking a 20 oz bottle, and the headache goes away immediately.
3 days this week I had to break down and drink a diet coke. The headaches do not seem to be as intense as they were in the beginning, but still there. My friend told me it took her 3 weeks before her caffeine withdrawal symptoms went away.
Another 2 weeks? UGH
Stop drinking caffeine. Period. Until you really quit, the headaches will keep recurring. When you get the headaches, do not drink caffeine. Remind yourself that the soda is the cause of the headaches, and they will keep recurring until you stop drinking caffeine.
I'm prone to migraines and finally quit caffeine a few months ago. Initially I coped with severe pain, nausea and vomiting for a few days. That was followed up by a moderate to mild, yet constant, headache that lasted for a week or two. I can't take OTC pain relievers for other health reasons, so I just coped with the pain.
And mind you, that was after completely, cold-turkey quitting. The times before that when I tried to taper or quit & backslid just led to recurrent headaches like you're having, and the craving for caffeine never subsided, so I kept sliding back.
Going cold turkey was painful, but it finally freed me from the headaches & from craving caffeine.
I was a caffeinated soda person, and for me, Sprite has been a good reverse-gateway drink to still have soda but not need caffeine. I also drank some chocolate milk for a while after quitting, which has a negligible amount of caffeine, maybe enough to take the edge off, but not enough to keep me a junkie. I don't drink it any more.
Find what coping mechanisms work for you. But the bottom line is that for some of us, caffeine creates a physical addiction that is going to cause us pain for the first few weeks of withdrawal. But it is oh so sweet not to be hooked any more. Don't give up.