Deceptions: Ascendant, Book 3 - Craig Alanson

Koren Bladewell now knows he is a wizard, but he is lost in the wilderness, cut off from help, and unable to use his powers. The enemy is poised at the border with an overwhelming invasion force, and it will take wizards, soldiers and a young, untested princess to hold off defeat long enough for Koren to be trained and grow into his power. But he might not wish to help those who deceived him and destroyed his life.