Remember that Cannestan comes in various strengths and for various uses. If thrush is caused by diabetes and not sexually, there is a different type of cannestan you should use.
Personally, I experienced a really nasty case after 'taking a break' from my diabetes meds (Metformin). It lasted for three months and the doc was not impressed when I told him it was not sexually caused. He gave me the one-off use tiny tube of cream, which sorted it out. However, my blood sugar continued to spiral upwards and I had to ask the chemist for the appropriate cream. Oh, heaven! I wish doctors would get some up-to-date diabetic training!
The chemist explained the different strengths and gave me 2% Clotrimazole (Canestan). But if you suffer often, maybe speak to your doc and request tests.
When you get diagnosed, there should be a sensible pack of information, explaining about how much diabetes loves fungus!
Hope you're feeling better soon