So from what I gather you were tested for allergies in the past? And you have quite a few? It seems counter-intuitive, but I also find my allergies are worse in the fall. First, if you are allergic to ragweed and molds, they are fall allergies. Espeically the molds which like to grow under fallen leaves. Like Sannah said, dustmites get worse in the fall/winter too. Spring allergies come and go quickly, everything releases pollen about the same time, you have a few weeks of misery, but then it's done. Fall and Winter, your house is closed up and the allergies build up, and it gets worse as time goes by.
It sounds like you have a continuous cycle of your allergies causing a sinus infection. When you are miserable it can be hard to tell the difference between them, which is why you seem to get different diagnosis from the doctors. The best thing is for you to prevent the allergies in the first place. Use the sinus irrigator. I know it didn't seem to help, but use it before you use the Nasonex. My allergist told me using a steroid nosespray without irrigating first is like putting oil on water. The spray will just sit on the mucus and never reach the nasal tissue where it needs to go to work. After a week, I think you will find the spray working better.
Second, ask about a different antihistamine, Clartin and Clarinex don't work well for some people. Allegra and Zyrtec offer some people much more relief.
If you are allergic to dustmites, get the dustmite proof covers for your mattress and pillows. Wash your sheets once a week in hot water, and the other bedding every other week. Go to Home Depot/Lowes and get a 3M or Dupont high effieciency furnace filter. This will help remove allergens from inside your house. And if you can, buy a HEPA air cleaner for your bedroom.