What If? The Afterlife of Severus Snape

Everyone wants to reach their happy ending, in this life or the next. After everything he has endured, Severus finally gets the second chance he wanted, the chance to undo one mistake, but which? With the help of a mysterious spirit with her own agenda, he has the chance to see worlds that could have been his own, but will that be enough to save him from his worst enemy: himself?