Long term sinusitis is often 1 or 3 things (or a combination)
1.) food allergies . You can get a food allergy BLOOD test (ask a holistic doctor - the skin pin prick tests that ENTs/conventional doctors suggest are not as accurate.
2.) Heavy Metal Toxicity (Mercury/other heavy metals). Once the heavy metals are removed from your system (through EDTA/DMPS chelations - some patients have to get their fillings removed first) Heavy metals can hold onto yeast/molds/fungus and make it much harder to get of. My long term sinusitis completely cleared up after doing EDTA chelations. If you choose to do this, make sure you do it with a holistic doctor you trust.
3.) Yeast/Fungus - high potent Oregano oil works wonders for this type of sinus problem!