Let's sort this out a bit:
You've had ON twice (I'm assuming this has been formally diagnosed, not you thinking you had it, but determined to have had it by an opthamologist (or other doctor)?)
You've had MRI's of your brain and spine. While lesions didn't show up in the brain MRI, there were lesions evident on the spinal MRI.
Your GP and opthamologist think you have MS (you have lots of other clinical signs to support their opinion).
Seems to me that you need to continue to see the neurologist, need to set aside your fear of being made to feel insignificant, and continue to get MRI's of your brain. And continue to tell the neuro of new or worsening symptoms, and continue to undergo testing. Clearly there is something wrong -- it's not part of "conversion disorder" to have ON, but I suspect the neuro is watching and waiting to see if you have new symptoms, or worsening symptoms, before finally calling it what it is.
If I were you, I'd re-schedule the MRI, and go through with it. Even if it does come back "negative", it needs to be in your record that you are continuing to have symptoms. You can ask your counselor to be in touch with your neuro to report that he doesn't think it's an emotional or psychological problem. Sometimes doctors talking to doctors are more credible than a patient trying to say the same thing.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....