Constant Headache, three years, no explanation. Anyone with similar situations/solutions?

I've had a headache for the past three years, for no apparent reason. I'm 17. It's constant from the moment I wake up to the time I fall asleep, and I've seen about 6 different doctors, received acupuncture and been hypnotized, taken Advil, been on three different prescription medications, as well as several other medications, and nothing has worked (excepting the acupuncture, which got rid of my headache for a total of 5 seconds following several weeks of frequent treatment. It returned immediately after those five seconds, and didn't go away at all after any subsequent acupuncture sessions). The pain ranges from just noticeable to throbbing with pain, and changes in intensity without warning and for no apparent reason. It started out of the blue in late October of my freshman year, and it's not a migraine or hormonal in any way. I'm confused and frustrated. Has anyone had similar experiences? If so, how were you able cure it?