Well, chronic sinusitis is common condition which usually means that next 12 weeks you will have swollen and inflamed sinuses. Of course this depends on the treatment options but usually 12 weeks is the best prognosis for this condition.
Since this condition can have many symptoms, and nausea, is one of them you are probably experiencing the worst case scenario. Nasal congestion is preventing you to breathe normally and pain is affecting not just area around sinuses but your nose, forehead and even ears.
Treatment of chronic sinusitis is usually reducing these nasty symptoms.
Did you consulted your doctor about this and what treatment did he recommended?
Anyway, whatever treatment option you are applying try to increase fluids intake and boost your immunity system with vitamins.