Help with my horse story?? I feel it's missing something?

What you're missing is conflict. Aside from the initial problem/conflict where Miles can't ride anymore, you don't mention any other conflict in the story. What is the problem here that needs solving? What obstacles do the characters need to overcome? It doesn't have to be anything major, but most stories have some kind of problem or conflict. It can be external if you want or entirely internal within the characters themselves, but without that opposition, you're not going to have a whole lot of rising action and who knows what the climax would be. There probably wouldn't be one. Find some sort of problem or challenge your characters need to overcome.