Hello~This could be due to a couple of things, the first thing that comes to my mind is "Candida" Candidiasis is a fungal infection caused by a yeast (a type of fungus) called Candida. Some species of Candida can cause infection in people; the most common is Candida albicans. Candida normally lives on the skin and inside the body, in places such as the mouth, throat, gut, and vagina, without causing any problems. Here is a link to one of many sites describing it and how to handle it: https://www.health.harvard.edu/a_to_z/candidiasis-a-to-z
Another thought that came to mind was a vitamin "B" deficiency, this can also cause the issues you are having with your tongue. I would add either a good, high potency "B" complex supplement to your diet, taken twice a day, or add "Brewers yeast" flakes to your food, this taste especially good on pop corn and cottage cheese. It is loaded with "B" vitamins and important minerals that can fight Candida such as "Chromium". You can get it at any health food store or store that sells it by the bulk.
It might be a good idea also to see your GP and ask for some tests, normally a stool sample will show Candida, but not all the time. One of the best ways to determine if Candida is the cause is to do an "elimination" diet, this is where you eliminate all yeast, wheat, and sugar products (one by one, this takes awhile) if you start feeling better, then more than likely, Candida is the cause. If not, then try the "B" complex and \or Brewers yeast.