What causes constant clogged/congested head feeling?

I know I get enough sleep (more than usual, because I always have this drowsy feeling in my head and I try to sleep it off, but it doesn't go away). And I don't believe it's caused by allergies, because I've tried several allergy medicines and they don't get rid of my symptoms. I don't really feel sleepy, I just don't feel like doing anything because my head feels clogged up. It feels like there is pressure in my upper head. I know the cause is not sleep related, because I always get a good amount of sleep. And I've had this problem for over 5 years, and I have it constantly, not just some days or for a few hours, but all day every day.