I feel that way all the time. It's especially worse when I have good days, where I'm able to function decently well, it's like, "Oh, well! Maybe it's not that bad actually and it's going away and I'll be able to do everything I want when I want in no time!" And then I overuse energy I don't have and crash and burn, repeat cycle infinitely lol
I also relate with doctors and testing. My old cardiologist dismissed my concerns that her lowering my med dosage was causing worsening fatigue because my "test results/data don't show that". Wtf does that mean??? Like clearly I'm experiencing it whether or not all of my labs, scans, and monitors come back normal (which they all do, consistently, very annoying and unhelpful). I've had multiple doctors now look at the bp and symptom log I keep and go, "Huh... Your symptoms don't seem related to your blood pressure at all, that's strange". Not very motivating, that's for sure. I've been called an anomaly a few times
It's hard to diagnose things that can't be explained or seen through medical testing. It's even harder to get others and yourself to believe in their existence/validity. Just know that your symptoms are very real because they are effecting you and that's all you need to know. Brush those other thoughts away like an annoying bug because they're just internalized ableism trying to make you doubt yourself and your understanding of your body. Doctors and other people can worsen these thoughts for sure, especially with medical gaslighting, but try and stay strong. You're chronically ill and can't do some things that healthy people can, and that's okay. Just because you know and respect your limits (leading to better well being and quality of life) doesn't mean you aren't still sick, just that you're getting better at managing it