My little sister (12) has had the same problem for a few years, she gets headaches every day and has difficulty concentrating.
She has ADD which is why she can't concentrate but the headaches make it worse.
We have been to doctors, had xrays, tests, MRIs and almost everything done and still can't target the problem, we even tried acupuncture therapy.
Go to you GP and explain the headaches for 3 years and explain the lack of concentration, through tests and what not they should be able to pin point the problem.
There is numerous reasons that can cause these symptoms so working it out with your Doc would be the best bet.
It could be something in your brain or could be related to deficiency of some kind, or tension in the muscles, there's just so many things it could be!
See your doctor