It is unclear what your doctor means by x-ray findings of "phlegm in my lungs". Green phlegm is always abnormal. You do not have to live with this condition. Green phlegm is not seen with asthma, although discolored mucus plugs can sometimes be seen with asthma. Also green phlegm is not a function of your age and the environment. Besides, you are still young.
You should seek another opinion on your x-ray, from another doctor, perhaps your doctor can ask a board certified radiologist to look at it. Green phlegm can be seen with a number of conditions including, but not limited to: pneumonia, bronchiectasis, bronchitis, immunodeficiency states, lung abscess, a foreign body, tuberculosis and cystic fibrosis.
Bottom line is that the cause of your phlegm should be determined by another doctor, more experienced in the diagnosis and treatment of lung diseases. Then you can get the appropriate treatment. The odds are highly in your favor.