There's a lot of good ideas there, Little Hawke. Sounds like acid from the very sour apple biting into the exposed roots, caused by gum recession. An ageing process which can occur at any age! And can be made worse by using your tooth brush as a saw. You have to keep on using the densitising tooth pastes: it goes away all right but then returns. How can you eat dreadful things like Granny Smiths! Try Royal Galas or Golden Delicious.
Good luck!
Janine