Why do I always have phlegm in the back of my throat?

For as long as I can remember, I've had phlegm in the back of my throat. I always have to spit it out every 5 minutes because it feels uncomfortable and it feels like it's clogging my throat. I've tried drinking water and everything but I always have this. I've had a CAT scan or whatever you call it but the doctor said nothing was wrong. What should I do about this?