The problem is with your physicians, not with you.
Turning yourself into a "good" patient for them is not going to help. Finding someone who is not like that-will.
And yes, it is hard as hell to find a physician who can deal with a reasonably intelligent and knowledgeable person, who has an illness that doesn't seem to fit the book. But, being such a person and having such an illness you really have no choice. And such physicians do exist, even if unfortunately they are not the majority.
Trust me, it doesn't make that much of a difference if they say they don't know what your illness is, or tell you that you definitely have myasthenia, but none of your symptoms really fit. Or decide that you are in remission, when the treatment they gave you led to a severe deterioration in your condition. Or even worse tell you that your illness is so severe, unusual and hard to treat that its not worth their time and effort. Or, who knows, maybe you are just so smart that you deliberately faked it, because having such an illness is so unlikely that it just can't exist. Knowing that you have a potentially treatable illness and your treating physicians don't seem to give a damn, was for me at least the hardest of all.
And last but not least, some of the symptoms you describe as "cognitive difficulty" could be due to a sleep disturbance, related or unrelated to your muscle weakness. I would suggest that if not done yet, try and get a sleep study. also, make sure they measure everything, not only look for oxygen levels.
And remember that modern medicine with all its unquestionable advances still has its limitations and can't always help all the patients, even those with known and easy to diagnose diseases. Many times it takes a lot of patience and ups and downs before you start to experience any true improvement.
Quite a few patients with known, diagnosed and treated myasthenia, suffer from symptoms which are not less severe than yours.
There are many tools and aids that can help you lead a reasonably productive life, despite that. So, alongside searching for medical answers, try to find what else can help.

As understandably as you say- As to your question what will nearly always make MG worse- physically using your muscles beyond their ability at that time. For one person it can be running and for another it can be walking a few steps, talking a few more sentences or trying to read. And for the same person it can be different at different times.
The hallmark of myasthenia is fatigable muscle weakness, which means that with repeat use of that muscle it becomes less effective.
The same cup you easily held in your hand a few minutes ago, becomes (just like in Alice in wonderland) as heavy to pick up as the whole table with the dishes and cups. A very reasonable distance to walk, becomes like climbing the Everest when you try to walk back. One day you manage to do everything you planned to do (within your limitations) and the next day you find it hard to even get out of bed, without someone helping you.