Have you spoken to your doctors about this problem? Two years sounds like a very long time for you to be still feeling like this. I have no experience with tongue cancer, my husband had oropharyngeal cancer and he had 7 weeks of radiation. That was enough to give him dry mouth. He carried a water bottle around and rinsed his mouth whenever he could. That’s a very basic way to deal with it but it worked well for him. Over time, it appeared to me that the dry mouth improved a bit. I hope that is going to be the case for you. But I would really want to let the doctors know/have a discussion with the doctors if I were in your shoes.