Mucus build up in throat

For years I have had mucus build up in my throat. Before I went to the doctor I decided to try Mucinex. After going through all the pills I didn't feel a difference. Then I went to the doctor and he said I had post nasal drip, which I didn't feel was a correct evaluation. So he prescribed a nasal spray and I used it to no avail. I wake up every morning having to cough up mucus and I deal with it all day. As much as I spit out mucus everyday, it feels like it just keeps coming back. What could be wrong with me? Thank you in advance.