After the return of Nightmare Moon and Luna, Celestia gets a tearful apology from her little sister. She gladly accepts it, and continues to rule Equestria as it was meant to. But she realizes one night that the cause of Nightmare Moon was her fault, not Luna's. She realizes she has never let out her feelings about that fateful day.

Princess Celestia keeps her feelings away, in a bottle to look at later. She can't focus on herself - she has far too many other duties to attend to. However, you can only ignore the guilt and pain for so long until it consumes you and breaks you.