Rach, I am so, so sorry for what you are going through. I can't remember if you've talked about this before, but can you go to a different GP - start over there since you have to be "referred" to a neuro? If not, it's really tough but you have to try to stay strong and work with him from where he's coming from.
I agree with others that it's a good idea to ask him about himself - what is HIS reasoning for diagnosing or not diagnosing MG? Could it help to tell him that you're on this forum and talk to many people in the US and other parts of the world who have been diagnosed with MG without having positive results from tests? That, instead, some people are diagnosed based on clinical assessments and response to treatment with Mestinon?
Have you been on the website,
www.myasthenia.org? They have a new booklet for health care providers that can be downloaded. It lists a lot of stats including info about people diagnosed with and without positive test results. I can't remember all it says but I think you might find some helpful info there and it is definitely credible and well respected.
I'm praying for you and thinking about you. Just remember you have strong supporters here and strong family support and let that help give you confidence and strength.
Take care,