Hi, I don't have any advice, only my own story to share... I was averaging at least 20 oz. of regular soda a day, (Mt. Dew mostly)...
I'm currently on my pre-op diet, with surgery only 5 days away... caffeine and sugar are both gone for me, too... I did have a bad headache the first 24-48 hours... but, so far, it's tapered off since... and more surprisingly, I haven't craved soda that much (probably since it's not in the house)...
for me, it was always about the sugar and the flavor anyway, not the caffeine... I never drank diet soda, couldn't stand it... which is why I'm having some trouble with Crystal Light, Propel, etc... don't really like the taste of artificial sweeteners... probably will move away from that eventually, to just non-sweetened beverages... but, that's not easy, either...
keep it up, and just try and battle through the headaches... if they persist or get worse, I would ask your doctor for advice... because, normally, there should be some relief on the horizon... but, it could take up to a week or two...
my (semi-educated) guess is that your body is suffering from more than just caffeine withdrawals... your body becomes accustomed to all the other dangerous chemicals in soda, too... especially diet soda... I imagine that those could be part of the problem, but will also flush out of your system eventually... good luck!
Doodle