Lots of Phlegm and closed of airways?

Im 13 years old and I have a HUGE problem. My airways in my nose completely close off, and feel stuffed up, but I can sit there and blow for hours without getting anything out. I can literally feel the phlegm running down my throat, and it is disgusting. Ive had this problem for a long time, though Ive never really noticed it until I started singing alot recently, in fact I have an audition for a huge musical tomorrow that Im worried about. I have been to many doctors and specialist, and I have absolutely NO alergies. What is the cause of it? What can I do to stop it? Anything that will work enough to clear out my sinusis for at least an hour tomorrow for my audition?