Rach,
Since he has requested the "long chat" take advantage of it. Go into with an open mind - not that it's mental, we all know better! - but so you can respond to him reasonably and rationally. DO take some information about being sero-negative and still having MG - point out that the MUSK antibody was relatively recently discovered to be MG related and the stats on that. But also ask him what convinces him that you do not have MG, or some other neuro problem, rather than a "mental" situation.

Make sure the info you take is from respected sources, let him know that you have done a lot of research and what you have found. Tell him if it's not MG, it is something else and that you NEED someone to keep checking - there are so many illnesses, conditions and tests! Let him know that it's not necessarily an MG diagnosis you need, but a diagnosis that makes sense with what you are experiencing, and you are not willing to stop at the old easy "mental" thing. There are too many other possibilities to be explored and you need someone willing, knowlegeable and able to help you explore them.

Even as I tell you to go with an open mind, though, you have to be prepared for the possibility that you won't be able to convince HIM to do the same. But no matter what, stay rational during the conversation. You've already hit too many road blocks in your search and you really need to turn this guy around if at all possible. If you give in to emotions, which is completely natural - I've done it!, you give them validation for suspecting mental issues. They often just cannot grasp the concept that this serious a health issue and change in your life can cause depression instead of depression causing the problems!

Best of luck to you, hon! Just stay "in charge" - firm, but reasonable. We'll be pulling for you!

Becky