Persistent bad breath and white-coated tongue for two months—tests are clear. What should I do?

Despite maintaining good oral hygiene, I’ve been struggling with persistent bad breath and a white coating on my tongue for the past two months. When I clean my tongue, the residue appears yellowish.

I don’t have tonsils, so it’s not tonsil stones. An ENT suspected a fungal infection and prescribed Nystatin drops, but they didn’t help. I’ve done multiple tests, including swabs of my nose, throat, and tongue, as well as a test for intestinal Candida—but no bacteria or fungi were found.

This has been going on for two months, and I’m out of ideas. What could be causing this, and what should I do next?