Hi. I Have Had Trouble Breathing For A While Now.

Hi. I have had trouble breathing for a while now. When I lay down or sit down for too long my nose becomes filled with mucus and has pressure in my sinuses. I've gone to many doctors and no medicine has helped we can't tell if it's allergies or what. But like right now it's so bad I cannot breathe at all through my nose. If you have any idea what could be wrong please help me. Thank you so much!