A lot of phlegm for years

I am 35 years old man. I have been troubled by phlegm issue since many years ago. First time, doctor said my nose water flows backward, after this issue resolved. I recovered for a while. maybe few years, then again, I have phlegm accumulating in my throat, airway little little non stop most of the time, making me need to cough it out or spit it out few times within an hour. Sometimes my condition got better, few hours no issue. sometimes were just bad. Then, another doctor said I am sensitive, have allergy issue, used nebulizer one time in the clinic. She gave me eat meptin and other medicines that time. Seems meptin works, at least for that time, but I have side effect like stretch mark by eating deltasolone 5mg (most likely). asthma medicine by that time given by the doctor. I din drink meptin for past 1 year. Just now I feel I have a lot of phlegm, I need to spit it out every 5-10 minutes, if I dun spit it out, I feel it will make me choking. I breath rapidly to inhale and exhale more air to make me more comfortable. (like asthma attack, maybe) Then I drink a tablet of cetirizine. Now after breathing hard a while, I feel more relieved. I feel myself pathetic in midnight, I wonder should I eat back meptin? or how? I even think I should go back to the clinic which I got meptin there, but now roadblocks are so many. I saw this website selling meptin syrup 60ml, need prescription. I plan to buy tomorrow morning, probably use the paid webcam service too. I dun know what type of medicine is suitable for me.