Take Back Magic by Casey Blair

Earth’s magic was stolen. She’s taking it back.

All Sierra Walker has ever wanted is magic, but the powerful mage who trained her as a child betrayed her, sending her back from a world full of magic to modern-day Seattle. So when he comes to her a decade later, Sierra glimpses an opportunity:
She steals his wand, and with it, she begins to steal back magic—for everyone.
In magical combat Sierra’s a force to be reckoned with, but she’s still on her own, her access to magic handicapped, with an entire world of mages now trying their best to kill her rather than share power. So when Nariel, a dangerous demon lord, offers his assistance, she can’t say no. But she also knows better than to trust her life, and her magic, to anyone else ever again—no matter how he ignites a part of her she’d thought long lost.
But as they race around the world to bring magic back while fighting for their lives, Sierra realizes there may now be two things she can’t bear to lose.