I've already been on a Candida/hypoglycemia/gluten-free or other restrictive diet, but I haven't improved at all. So food must not be a reason for my fatigue.
False. General diets may not work because they are too broad. The only way to find out is by doing an individualized food test after which a tailor-made diet will be created for you.
My doctor says it's all between the ears.
This may be true in some, but most people can tell quite well whether something is psychological or not. If a person feels it's not between the ears, there's plenty of reason to keep on looking.
There is no one cure for ME that works for everybody.
This is absolutely true. Chronic fatigue syndrome is very complex and what works for one, may not necessarily work for another. If conventional testing has not led to anything, the least that should be done is eliminate immune-undermining factors such as food intolerances and damaging environmental impact.