As a kid, I have been extremely bad in sports, could never run like other kids, but there was nothing to suggest any medical problem. I was nicknamed "the professor" by my friends, because I always preferred quite activities.
My father and one of my brothers are "less fit" and it takes them a longer time to recover from viral infections. But both run, play football etc. with no significant problems.

20 years ago, following Hep. B vaccination I had "MG like symptoms" but those resolved on their own within a year.

15 years later (after leading a very active normal life) I ended up in the ICU with what appeared like a myasthenic crisis. As I had completely normal tests, this diagnosis was disputed by the neurologists.
My illness was eventually found to be a rare form of MuSK MG, possibly combined with congenital myasthenia, mostly involving my respiratory muscles.
Some of my studies also suggest mitochondrial dysfunction (which can be part of the MuSK MG or not).
The complexity of my illness led quite a few physicians doubt its existence.

Like you, I had to figure out quite a lot on my own.