Is it possible for a "mild" head injury to have serious long term effects?

My concussion was over 10 years ago, yet I am still feeling the effects - mostly in the form of bad headaches and extreme mental fatigue. My symptoms get that much worse when I don't get a good enough sleep. For me a "good" sleep is at least 9 1/2 hours without interruption.

This injury has really affected my life. Because of my mental limitations, I work as a security guard; this fact is shocking to people who went to school with me. I have, or at least had, a very high IQ.

I read on Wikipedia that PCS symptoms almost always resolve completely within a year. Why then am I still suffering these effects? I can't even read the newspaper without having to stop what I'm doing.