scare her, keep scaring her, eventually she'll get tired of it, Is she spooking at anything in particular? Like trash blowing across the trails? Leaves? Shaking bushes,?
What I do is if horse spooks at soemthing I make him go back and then do circles round and round whatever it was, horses hate circles, eventually they will connect the spook with having to walk in circles. Just going to take time,
I have no proof, but I see alot of horse really liven up in the fall and do alot more running around. My theory is horses evolved from herd animals, herd animals all migrate south in the fall. Something in a horses brain is telling him to head south, but he is stuck in a pasture, so he gets stressed.