The Mirror and the Curse by J.M. Stengl

This summer, Princess Eddi intends to win both Prince Fidelio’s heart and the Faraway Castle Cup!
A summer of training for the race at the resort with her winged horse and Kai the dwarf sounds ideal. Better still, Prince Fidelio decides to join her . . . so her summer plan is a done deal. Or it would be if Fidelio would behave like a sensible boyfriend, and Kai? Sidekick dwarfs are not supposed to be confusingly attractive.
A series of mysterious magic attacks merely adds to Eddi’s trouble. But who is out to get her? Her nemesis Raquel is the obvious villain, but she doesn’t have magic. Eddi’s annoyingly perfect stepmother does, however. Worse still, there’s rumor of a dangerous magical artifact on the loose . . .
Can Eddie power through one obstacle after another and achieve both her summer goals? Only if her heart will cooperate!