Ever by Gail Carson Levine
Kezi's mother is deathly ill. Her father is desperate to save her, so he makes a vow saying if his wife recovers, he will sacrifice the first person to congratulate him on her health. In order to save her aunt, Kezi steps in to be sacrificed. With only a month left to life, she is in despair.
When she meets Olus, the Akkan god of the winds, she immediatly falls in love. Olus, also in love, desperately thinks of a way to save Kezi from certain death. He takes her to his home and together, they must go through tests to prove Kezi worthy of immortality. Kezi's days are numbered and the tasks seem impossible. Can she pass her test in time?