Frostbite (Destroyers, Book Four) by Holly Hook
Ever since her mother left her, Sophia's teenage years have been plagued with another presence that lives inside her-an angry old hag that freezes everything around her whenever she gets upset or angry. She cannot tell her grandmother or anyone else of the other, even though it can rise up and kill everyone she loves in an instant.
But everything reaches a tipping point when she learns that her boyfriend, Shane, has been spying on her best friend, Callie. It seems that Callie has some secrets of her own--she can control the wind and the waves. Worse still, storm clouds fill her eyes sometimes, and Shane has taken his video of her to the Internet. When Sophia learns of Shane's betrayal, the hag inside her rises up, nearly killing him and exposing her own secret.
Things worsen when she runs to Callie, only to discover that her best friend is a Tempest--a human hurricane--and that an insane goddess is after Callie's family. Sophia is quickly discovered by Andrina...and the hag inside Sophia sees its opportunity for freedom. The other inside her wants to join Andrina and help end the world. If it gets out, Sophia will face her own destruction.