Cross-Country Gallop (Book #3 in the Horseshoes/Kestrels Series) by Patricia Leitch

Sally Lorimer and her best friend Thalia love to ride together–most of the time. Then they sign up to ride as a pair in the local horse show and their troubles begin. When they practice, Thalia and her roan, Tarquin, go much too fast, while Sally and her dapple gray, Willow, can’t keep the pace. Will the horse show be a disaster or will Sally find a way to turn things around?