I was riding a school horse today who for some reason was being really skittish. He was spooking at one of the doors down the side of the school but it wasn't anything I couldn't handle. About half an hour in he started really playing up but I kept my leg on to keep him going forward. This was fine until I got him into canter. I was going large down the school and he was REALLY picking up speed so I tried to half halt - instead he came to an emergency stop! I wasn't expecting this at all so obviously I came off right over the top of his head

.
Was just wondering in the future how to deal with horses like this? I hate riding horses that spook, after having had an accident on one two years ago I really had my confidence knocked. I definitely wasn't too heavy on the reins. What do you guys suggest?