I've been smoke free for over 150 days but, I haven't noticed any positive changes. I never coughed up brown or black flem. I always ran and smoked while in the Air Force for 20 years. I only change I experienced is my daily glucose numbers rose after I quit and they should remain higher than normal for up to three years from what I have read. I have had to inject more insulin and have gained 40 pounds in six weeks due to the increase of insulin at each meal and when I inject the long acting insulin twice a day. I do not see a valid benefit to quitting after being smoke free over 150 days.
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I have only been smoke free for 34 days and I've noticed 2 measurable benefits already. My oxygen level went from 94% to 99% within the first 3 weeks and I've already noticed that when I walk and hike uphills (I live in the mountains), I do not get out of breath as quickly nor does it take as long to recover as it did. I expect this to improve the longer I remain smoke free.
I never had the coughing associated with smoking though I must admit I have developed a mild cough now as my lungs begin to heal themselves. I could go into a long list of the medical reasons and benefits but I am sure you have seen those. Among those is the fact that quitting smoking lowers blood sugar levels unless you are diabetic, then it can have the opposite effect. Smoking does contribute to diabetes.
It sounds you need to talk to your doctor about what you need to do short of smoking again.