i Empathise with you, OP, and hope you can get some answers soon.
your title says your GP thinks you’re crazy and several posters have advised you to change GP but you haven’t said what he or she has said, or done, to make you think they think badly of you.
GPs are often getting a bashing (and often rightly so) but this GP sounds like they’ve taken a really sensible approach of systematic testing, treating deficiencies, then reviewing treatments. It doesn’t sound as though the GP has dismissed any of your concerns. And has not been quick to throw a diagnosis of exclusion at you, presumably because they want to ensure all other possibles have been excluded. Unless there’s something you’re not telling us about your interactions with this GP, then I’d stick with this one because they’re clearly taking you seriously.
you have lots of good ideas from posters above, make a list and discuss with your GP at your next appointment. A good GP will either explain, with evidence, why/how they’ve ruled something out, or agree to further testing if not ruled out.
Your symptoms do sound auto-immune, and lots of these conditions can be difficult to diagnose because there isn’t always a specific test and the tests we do have can be negative even when you do have the condition. Diagnosis needs someone to look at the big picture of your symptoms and their progression/patterns, blood test results, and risk factors. In addition it can take years to develop the full gamet of symptoms that point to the specific syndrome you have.
To Add to the ideas from posters above, I would be inclined to ask for an early morning cortisol level. It certainly isn’t a first line test, but I think worth checking here.
Edit to add: don’t disregard stress as a cause, we do not know the depths of the intricacies of the connections between mind and body, and certainly stress can cause extreme physical illness. Aside from that, being proactive in managing stress will be massively beneficial even if it isn’t the primary cause of your symptoms. All the best.