I Have Been Coughing Up Long Sticky, Yellow Strings For

I have been coughing up long sticky, yellow strings for months. They get caught in my lungs, and I feel as though I'm going to quit breathing. My nebulizer and rescue inhaler no longer work. My doctors don't know how to help. This is affecting daily living. My lungs are tired. Lung x-rays don't reveal anything. I also have grown nasal polyps over and again.