I've just gone through the same thing. The first day after I quit I already had a tension headache AND a sinus headache, and seriously considered just giving in...but then I decided I might as well get all the headaches over with at once!
And I did use Excedrin Migraine after that first day. I figured the caffeine I was getting from it at least wasn't from soda, so I wasn't continuing on in my bad soda drinking habit by taking it. It helped. And today I was sooooo drowsy at work (4yo DD was up with nightmares last night) that I broke down and got a regular caffeinated Caramel Macchioto at Starbucks at lunchtime.
My problem now is that I really don't care for plain water. There's only so much of it I can drink in a day. I really like Propel fitness water (flavored), but it's awfully expensive. I've been drinking Country Time lemonade, but it's high carb. I'm off nutrasweet along with the caffeine. Anybody have other suggestions for what to drink all day?
(BTW I've been half filling empty Propel bottles with water and freezing them, and then filling the other half with water or lemonade. I do like the plain water better ice cold than luke warm.)