Help me find this book I’ve been looking for years

Help me find this book I remember from high school, it has been driving me crazy for years

There is a war campaign going on at the start of the book and one girl has hidden (light/sun/good) powers who has either worked to keep them hidden or didn’t know that she has them (can’t remember which). She gets caught with her powers by the (King/Lord/or some other fancy name) and is taken to the castle where the king tricks her into falling in love with him to use her and her powers or kill her. The girl figures out his true motives and runs away with a new love interest. They hide out for a while and eventually get the support of the kingdoms people. I remember a scene where the girl is ridding a horse through town where she finds a bunch of banners the people had hung to support her. After a dramatic scene involving a river that is too fuzzy for me to remember details, the king ends up dead and we learn his real name (I think it was something fancy sounding starting with an X) and he wound up having some tragic story that made readers feel bad for him.

Clues:
-It was in a high school library so it was geared towards teens.
-It was avalible in a school so it had little to no spice and no overly graphic death scenes
-I read it in my Freshman year of high school so it had to be published before 2019-2020