Complex sentence: Trained by the former jockey, Kay Spence, who began by training claiming horses and ended up training the winners of five Kentucky Oaks, said that Princess Doreen represented the pinnacle of his success.
Split into simpler sentences: Trained by the former jockey, Kay Spence, began by training claiming horses and ended training the winners of five Kentucky Oaks. He said that Princess Doreen represented the pinnacle of his success.