I also have constant 24/7 head pressure. I have no diagnosis but only a couple of my 15 or so symptoms resemble some aspects of migraine/aura phenomena. The important thing to remember is this, usually, a migraine is NOT constant. However, my head pressure is. I have some degree of it every single minute of my life, and have for 6 years. This leads me to believe that migraine is not the primary cause.
I've recently started to examine the original idea, that it's my sinuses. But with no insurance, I can't get testing done.