Canestan will not treat oral thrush it is only for the nether regions.

But there is a treatment you can get from the chemist but I can't remember the name of  it   --- Sorry.

Also Corsodyl mouthwash is excellent for oral thrush and I found it very good.

I think you need to have a chat with your local chemist.

Hope you get it sorted soon, I know how you feel as I had it and it so uncomfortable.

Dawn x