There's a HUGE difference between saying "we don't know what's wrong and we're sorry but we can't help with medication, but sometimes there can be mental things that help" and "we've diagnosed you with a mental issue, your body is fine." The latter is way more dismissive and angering, and assumes that we know a lot more than we do about the physical systems in the body.

Neither of us were in the room, but it sounds like the doctors did the former. They diagnosed with FND, a real physical disease (although kind of a catch-all diagnosis with no tests or treatment, like CFS), and recommended that therapy may help.

Say that then. "I don't know what is causing this. I've ruled out everything I can, you may have CFS or psychosomatic symptoms, but I can't test for either. Here are some resources on both. Psychiatry has helped x% of patients with your symptoms, I can't guarantee it will help but here's a referral."

Much better to say "I don't know" than to give a confident diagnosis just because you've run out of things to test for.

There is no simple lab test for CFS, and there are no miracle treatments, but it can be diagnosed, and some of the symptoms can be treated.

For diagnosis, there are clear diagnostic criteria, including the presence of post-exertional malaise, which is very easy to recognize simply by talking to the patient. (“I went grocery shopping last week, and I didn’t think I’d overdone it, because I felt okay that afternoon, and I had a full night’s sleep, but I woke up the next day feeling terrible and I could hardly get out of bed all day.”) There is a lab test that can be done for it, but it essentially consists of making the patient “crash” by exercising two days in a row and observing how much worse they are the second day. It’s not recommended for diagnosis because it’s harmful (it can take months to recover from a bad crash) and, again, not necessary.

For treatment, there are a few supplements and prescription drugs that can help each of the symptoms. Many patients experience dysautonomia (it’s in the diagnostic criteria) and there are several treatment options for that (beta blockers, drugs to raise blood pressure, drugs to encourage sodium retention, and others, in addition to non-pharmaceutical options like increasing salt and water intake). The fatigue itself is harder to treat, but a few supplements have been found to help. Cognitive dysfunction is a major problem that is definitely treatable (with ADHD and dementia drugs); that alone is enough reason to get diagnosed and treated.