Horse Riding problem - needs sorting; fast!?

I left my last riding school because I had experienced a bad fall and lost all my confidence I had gained over five years. I moved and was very, very happy at my new riding school; I still am. Then this new horse came along, Gilby. I was looking forward to a new ride - I climbed aboard and all was going fine until I asked for canter. He sped up into an extremely, unbelievably fast trot and I felt completely out of control,having flashbacks of my fall. Now I feel the sickness and dread Agni like I felt at my old riding school; help, I love this place! What can I do? I feel paranoid but I adore horses and my riding! I want to improve!