I had the same problem, due to radiation though. Miracle mouthwash was used first, like you it finally quit working. I then was given a thick solution, like thick syrup. It had a pain killer/numbing agent in it. I would swish this around, hold it for awhile, then I would take a few bites of what ever I was eating. Repeat. Also, plain old chloraseptic worked pretty good. Take a few squirts, then take a bite.
Also, my oncologist told me to mix a solution of peroxide and water and rinse with it. That heals them up pretty quick.
I know it's miserable, I lost 50 # during both times I was treated and couldn't eat.
Best of luck to you as you conquer the monster!