Most likely you have an allergy to something causing the mucous.
Have you tried an anahistamine such as Zyrtec? If not, give it a try, works well.
Mucinex (plain) thins mucous also. My doctor told me to take Zyrtec and Mucinex, worked well for me.
At night when I have lots of mucous I take 1/2 teaspoon plain Robitussin at bedtime, nothing to drink after. This helps thin mucous in throat during the night.
Halls Cough drops work well also.
I am not a medical person, speaking from experience only.