Reawakened Flames by Lindsay McCafferty

Aria, the crown princess of Torrannon, has the world at her feet and a bright future. Then she develops anxiety and depression and finds herself trapped in a losing battle that seems to only have one tragic path of escape.

Landa, a phoenix, has an embarrassing accident, and her confidence diminishes. She then loses the one ability she wields as a phoenix—her fire. 

With an invasion against Torrannon looming, Aria and Landa must save the kingdom from utter ruin. But first, they will have to rise above their inner struggles in order to survive an encounter with a far more sinister evil.

This is the first book in the Sparks Shall Rise epic fantasy series. A tale about learning how to believe in yourself and how to find the smallest spark of hope, even when you feel as if you are drowning in darkness. Contains content dealing with anxiety, health anxiety, depression, obsessive-compulsive disorder, and suicidal ideation.