Sorry if you've already tried this, but have you tried taking methyl B12? If so, did it help?

I used to have the same symptoms as you, where I dreaded even picking up a pen or using a computer mouse, and my writing would be messy because I had no grip. Then I started taking cyano B12, but now I am going to move onto the superior methyl sublingual B12.

I think a nerve conduction test would be useful.