For years my sister had a very painful red and cracked tongue and a sore mouth. She was prescribed various medications but none of them helped. Then she had her amalgam fillings replaced with porcelain, but still no relief.
One day someone suggested it might be caused by food additives namely sulphites. She eliminated all foods with E numbers (E220-E228) and since then she's had no more sore tongue or mouth.
Very occasionally she'll have a bit of a sore tongue but she can usually trace it back to something she ate in a restaurant.
Foods to Avoid
Burning tongue-Mouth issues: When you have burning tongue or canker sores on tongue or in mouth. Do not drink any orange juice, ketchup, sauce etc. The best cure I have found is to rinse your mouth will Karo syrup. It seems to neutralize the acid. Also 2% hydrogen peroxide.