I had a post viral illness this year after glandular fever. The post viral illness is related to chronic fatigue and if it hadn't resolved I would have been diagnosed with chronic fatigue. I had every test the doctors could think of and I was seeing specialists, yet every test said I was a bill of health. It's incredibly frustrating to have medical practitioners not believe you when the illness is ruining your life.
In practice, living the years I have with my wife, the answer seems to usually be "assume that you, as a doctor, have much more knowledge about your patient's experience and that because no test you know how to do dings 'yep, a problem', that the patient in fact _does not_ have a problem, that their claims of being in enough pain to want to die at least a few times a week are made-up, that they've lived this long with their 'must be just in their head' problem that surely it's not so bad and they can keep doing so".
From a purely probabilistic standpoint, they might be right.
From my wife's perspective, they are torturing her, hopefully not to death.
What _do_ you do?