I used to get canker sores all the time. Back of mouth, top, at the furthest edge of there, on the side.

They would be painful, and then slowly heal. Well, I have had one for over a year, it never healed. What's up with that? It is hard in the center and slimy all around and moves side to side when I touch it. Feels a bit like a tiny pomegranate seed. I regularly take heavy duty supplements (highly effective zinc, oregano oil, etc etc.) and the darned thing is still there.

Why and how to get rid of it?