Ash Princess (Ash Princess #1) by Laura Sebastian

When her country was conquered by the Kalovaxians, Princess Theodosia was kept as a hostage and a tool to keep her in check. In the 10 years since the conquering, she has been publicly tortured many times and has been forced to give up her language, her people, and even her name. She thinks she's been defeated, but when she gets a spark of hope, she realizes how much she still has to fight for.