Among Shadows (The Heir of Light & Dark #1) by Aline P Mora:
 
I LIED, I KILLED, AND NOW, I’VE LOST EVERYTHING.
THE ONLY THING LEFT IS FINDING OUT IF ANY OF IT WAS WORTH IT.

In a world where powerful men hold all the control and magic is the ultimate privilege, Aila grew up with two secrets: spy to a powerful man and the only wielder of Darkness! She was groomed to kill and to lie, and she learned, from a very young age, that her own power could become her weakness. But her latest mission will either force her to reveal herself or to betray her only friend!

For years, she enjoyed the luxury and safety of Niram’s palace, trading in secrets and whispers whilst keeping her own power hidden at all costs. But, as she crosses path with a mesmerizing captain and a brewing rebellion against the power she served her whole life, she will realize there is more forces at play than she ever imagined. And she will finally have to learn exactly what her power and herself are really worth.

As her whole world crumbles down and her secrets are revealed, Aila will need to face her own mentor or risk losing herself.