I've had constant head pressure for almost 8 years. Mine is 24/7 but it doesn't usually cause pain, just a lot of discomfort and aggravation.
There have been a number of folks who've been to this board and have the same thing. Unfortunately head pressure does not fit into any kind of diagnosis other than "headache" usually, and most doctors--even neurologists--have no idea what to do about it. They can offer you migraine drugs, and hope that the drugs work for you. That's usually about all they can do.
Some people have been helped by such medication, others have not been. I tried Topamax and it didn't work. I'm not sure what my next move will be.