I have had sinus pressure and brain fog for three years on and off now. what is causing this, and what can i do to treat?

Sinusitis?: You have had these symptoms on and off for three years and never consulted your Doctor? I think if not then you should consult your GP . You need a detailed history and Physical Examination . May need blood test and / or imaging like X-rays and/ or Cat Scan and other tests if indicated . You seem to be getting bouts of recurrent sinusitis and it needs to be investigated and treated appropriate