Hi, Lily, and welcome to NT! Jo55 is quite right about many of the more unusual conditions being frequently mis- or un-diagnosed. Read through some of the posts on the Myasthenia Gravis forum.
Even if you do not find any answers here, you will for sure get a lot of support and sympathy. Many of us have been or still are in diagnosis limbo. And that is not a very friendly place to be. It can get awfully cold and lonely there. Especially when the doctors are telling you it is all in your head, but you know very well it's not!
Hugs,