The inner lining of our sinuses as well as the respiratory tract has lining of small projections called cilia which beat in a definite direction and help in bringing out the dirt and mucus. This is called muco ciliary clearance. But the smoking has damaged the ciliated epithelium and hence this clearance is disturbed. Hence you need to cough out the thick mucus instead of it automatically coming out even after you have quitted. This is also responsible for the sinusitis and the infective agents do not get cleared out from the sinuses. Since the lungs have also been damaged to some degree while you were smoking, you sometimes have respiratory distress or shortness of breath. It will take significant time to recover and will recover only to a near normal stage. You will have to be patient till then. Steam inhalation on regular basis may help.