Sooner or Later by Lauren Brooke

Heartland is a horse farm nestled in the hills of Virginia, but it's much more than that. Heartland is like no other place - it's a place where the scars of the past can be healed, a place where frightened and abused horses learn to trust again.

Amy's dreams have come true. She is working with damaged horses and showing in high-level jumping competitions. It's all because of Storm, the talented dapple gray thoroughbred that was a gift from Amy's father. But having it all isn't always easy, and Amy is realizing that she can't keep up with her work at Heartland and dedicate herself to the show world. Storm needs more attention - so do the other horses. Sooner or later, she'll have to make a choice.
 
- Goodreads Summary