I've had thick and excessive saliva for months now with mouth pain, trouble swallowing, nausea and belching. No visible irritation in the mouth.?

See a GI specialist. Sounds like gastric reflux, or something related. See GI md, if nothing, then a dentist.
Saliva. This sounds like something you really need to see a doctor for. There is no easy answer to that other than a good physical exam and a good history.