My friend is going through a similar situation. His mare tested negative for EPM but he still has no definitive diagnosis aside from some possible neuro damage from a toxin or an injury of some sort.
She wasn't as severely unbalanced or uncoordinated, though and she seems to be slowly improving with physical therapy. But it is probable she will never be ridden consistently again. She will be probably be a one time and retired as a pasture puff. (two vets have said breeding would be fine if done with AI instead of risking a mounting stallion.
Riding: The art of keeping a horse between you and the ground.