I am extremely sensitive to Mestinon dosing. The tablets at 30mg five times a day caused esophagitis, gastritis and duodenitis among other side effects with rebound as it wore off. I took generic liquid Mestinon 2mg (equivalent one thirtieth of a tablet) with good effect several years ago but stopped because of cost.
I now take 6mg generic liquid Mestinon (equivalent of one tenth of a tablet) every three hours as any more causes the side effects you are having. I was able to get the insurance to lower the tier so I did not have to pay $830 for eight ounce bottle but only $20 a month. The MGA patient assistance fund might help you pay for it. The studies recommend thymectomy even without thymoma (read New England Journal of Medicine article from several years ago you can google) not must medical therapy.
So you see not every one need the same Mestinon doses. If I ever had a full tablet I think it might have done me in.