Based on your description:

"Historical novel published before 2015. Male protagonist. While in disguise he is noticed by a girl. Later he finds the girl being threatened by her brother for allowing him to escape. A sorceress later reveals she taught his close companion how to cast a spell. Near the climax he reveals a curse to his love interest and tries to remove it, but it sticks to him. In the end he is encouraged to join hands with his love interest."