I've seen what jumping too high, too early can do to a horse, ErikaLynn. I OWNED a horse that had that happen to him. His owners started him over 2-foot fences as a three-year-old. They did the same thing you're doing: showed him over the summer (but with one schooling session during the week in between shows, which were once a month). As a six-year-old (when I started working with him), he had such severe arthritic changes in his hocks that you had to warm him up at a trot for a good 15 minutes before really lunging him. In the winter, you had to warm him up at a walk and trot for almost thirty minutes before really lunging or working him or else he could barely walk afterward. He's now a nine-year-old who is relegated to riding light trails sometimes once a week and his right hock has already fused due to the arthritis, which a vet has confirmed was because he started jumping too early. This was a horse that LOVED jumping. If you had a crossrail set up in the arena, he wanted to go over it, arthritis be ****ed.
I'm not trying to preach at you. I'm just trying to show you that even though you may not think you're doing any damage to the horse, chances are that's exactly what you're doing.