As Mike pointed out, Mestinon often has little or no effect on ocular myasthenia. Most often it is steroids that do have an effect.

I presume that the doctors have ruled out other possibilities such as palsies.

Have you tried patching one eye to see how much rest can help that eye. At least then with just one eye in use at a time the double vision should be gone.

If it is myasthenia, then as far as I know there is no way to deter progression. With some people it happens, with others it doesn't.