Chasing gold / Catherine Hapka.

Enhanced descriptions from Syndetics:

When all of Haley's time goes to training her pony, keeping an eye on the prize causes problems in this third book in a contemporary middle grade series in the tradition of Marguerite Henry's Misty of Chincoteague .

For months, Haley Duncan has been saving up to ride with her pony Wings in a clinic with a world-famous event rider. On top of that, she wants to make sure Wings is as fit and ready as he can be, which means extra trot sets before school and training and cleaning tack all weekend. So what if Haley's friends complain that she doesn't have time for them anymore? So what if she has to miss her very first ever school dance, or skip a favorite family tradition, or leave early from her BFF's first boy-girl party? Everyone will get it when they see her in the Olympics someday. But when Haley's overscheduled existence and exhaustion causes an accident, she may have to say good-bye to her dreams...