If GP won't refer you out to specialist , personally I'd dump that one and move on.
Could it be the GP wants to keep a regular customer???

I'd search around and not stick with any dr that isn't being a health care partner with me.

You may even need a pain management dr if there is nothing that can be done to fix the cause of the pain.

I don't know if it is all due to MS, or if any spinal issues or other conditions are involved.

Don't stay with an unresponsive dr.