look man im going to be honest, no one is going to help you, none of the doctors know the cause or care enough to figure it out. find yourself a good and kind plastic surgeon and get him to give you botox until you feel normal. i know you have already tried it but everyone is diffrent, gland production is different, gland size is diffrent, botox concentration is different, location of injections are diffrent. one way or another the botox will eventually shut them down. your parotid and submandibular glands generate 90% of your saliva, if you inject them enough your mouth will finally feel peace. you have suffered with this long enough my friend