For 18 Years I Have Put Up With A Feeling

For 18 years I have put up with a feeling that something is stuck in the back of my throat. This condition started after I had a cold in the Spring of 1997. Sometimes I can cough up thick mucus. This feeling is more noticeable when I lay down or when I wear any shirt that is snug against my neck. No doctor I have seen (15 so far) wants to find out what is wrong. What could be causing this condition?