Ok by sinusitis do you mean pain?

That is not the medical definition of sinusitis at all.  The medical definition
of sinusitis is objectively-measured inflammation or infection of the sinuses.

If your CT scan is clear,  how do you know that you have (true) sinusitis?  I
am not saying that you do not, I am just wondering........

Have you been checked for food allergies? You could try to eliminate
various foods and see if it makes a difference for you. Especially stop
milk and dairy products (do for 2-3 months to see a difference).

Do you eat a healthy diet? The reason I ask is many people in the 20's
live a hectic life and don't eat regular healthy meals. You could try
reading up on Dr. Weil's web site www.drweil.com. He has several books on
healthy eating and lifestyles. Take vitamins too they help me greatly.

As for allegy skin testing, it is not foolproof. Keep a symptom diary if
you notice that you are more congested a certain time of year or in
certain environments write this down and bring to an allergist. Also you
could try making sure your bedroom is totally dust free. Get dust mite
covers for mattresses and pillows. Free the bedroom of clutter (books and
misc items), get rid of heavy curtains and keep dust free. Run a good HEPA
filter at night. Use a good humidifier in the winter. See if makes a
difference. Reason I suggest this is year round congestion could be sign
of dust and dust mite allergies. Read about making your bedroom allergy
free at:

http://www.allergybuyersclub.com/allergy-center.html

You may want to get a complete physical and make sure they check your
thyroid levels which I bet they don't if you are male.

Finally do you have pets in the home? May be could be allergic to them.