I am so sorry you are having such a struggle. I have seen so many posts by MGers or possible MGers in the same situation. Please don't be annoyed that I have cut and pasted some of this from previous similar post. But here goes:

I am seronegative too. I was diagnosed by a single fiber EMG. I have a couple recommendations for you:
1. Go see a neuromuscular specialist at a large teaching hospital that specializes in MG or diseases of the neuromuscular junction. I go the the neuroscience center at Jefferson University hospital. MG is very rare and most neuros have never seen it. And MG is called the snowflake disease because it presents differently in every patient. So maybe your neuro has seen it a couple of times before, but maybe he is judging you based on just those couple of patients. Going to a teaching hospital where they see it all the time is very important especially if you are not the typical presentation like me in being sero-negative.
2. Statistics show that only 85% of MGer EVER show positive antibodies. And new MG patients may be seronegative at first and then turn positive later. You don't want to have to wait months or years for treatment waiting for the antibodies to be positive.
3. There are several different antibodies to the neuromuscular junction. ACH antibodies is the most prevalant-the commercial blood test done. Musk is probably the next highest. But there are others that have been identified in research that they do not even have commercial tests for. Teaching hospitals are more up in the current research and are more aware of these cases.
4. The single fiber EMG is the definitvie diagnostic test for MG no matter which antibody you have. This test can only be performed and interpreted by very experienced neurologist that you will find at a large teaching hospital.

You as the patient should not have to "prove" anything, although MGers are often put in this situation. I am hesitant to say exaccerbate your symptoms so the doctor can see them. You can put yourself in a dangerous situation if your breathing muscles are affected. I know for myself that after more than normal physical activity, I can usually get through the day but actual show the worse exaccerbation 24-48 after the activity. The resistance test are only a subjective measure, unless they have a baseline of where you were before MG, I not sure how well they can be interpreted.

My first neuro simply said the antibodies are negative, it is not MG. The neuromuscular specialist at a large teaching hospital did an SFEMG and had no doubt it was MG. The key to the testing is repetitive testing to see the declined response as the muscle repeat the test. I do not think a lot of patients who go to doctors that are not MG specialist get adequate repetitive testing to see that.

I have to travel more than a 3 hr round trip to see the neuromuscular specialist. But it is well worth it. Once you get the definitive diagnosis, your local doctors will me much more accepting of a definitive diagnosis from a highly acclaimed specialist in the field that you will no longer get the scruitiny. The local doctors can manage you based on the neuromuscular specialist recommendations.

Good luck and let us know how you do,
kathie