Hi, Antsy Pants! Welcome to the world of unpredictable MG.
It's good that you found a neurologist who was thinking clearly.

I'm pretty tired of them pulling that, "It's all in your head" nonsense on women who have very real symptoms.
MG doesn't like to be pushed. It will push you right back, possibly into a MG crisis. You do not want to go that far, because it will take much longer to recover from that and they often do more lovely drugs (IV Solu-Medrol) in the hospital, too.
No one "likes" to change their life, but it's necessary with MG.
Did you have any side effects of Mestinon?
I'm not questioning your diagnosing doctor, because your symptoms sound like MG, but did he even do a Tensilon test in order to "prove" you have it? Did they do a Single Fiber EMG?
Since you have breathing issues, it's important to find a good pulmonologist. They can assess where you are right now and try to determine what your "normal," or baseline, would be. They should do MIP and MEP in addition to the regular PFTs/breathing tests. MIP and MEP show how well you're doing at breathing air in and out, and they are specific to a neuromuscular disease.
An oximeter can be useful if you have breathing issues on a regular basis. When breathing is worse, your pulse might go higher as well (the heart is working harder to get you oxygen).
A pulmonologist can also do an overnight oximetry, to see how your breathing is while sleeping. It can be much lower if you're not doing well, and you might need a BiPAP or oxygen at some point.
Some drugs can make MG worse, as can a lack of sleep, getting too hot, infections, surgery, etc.
www.myasthenia.org
www.mdausa.org
It helps to read up on MG, because there's a lot to know.
Is there anything else you need to know or want to vent about?
I hope you can find a good balance between doing and MG. It takes some finesse and, even when you think you know what you can do, MG can always surprise you!
Annie