Regret, pain, sadness.
The princess spent each day harboring these feelings, torturing and agonizing. For a thousand years, the princess was dead inside. Every day, every wretched day, she woke up, hoping it was all a dream, that her sweet, beloved sister would be nagging her to get up and play with her. Every hour, every minute, of those thousand years was a harrowing curse.
The princess knew, she knew in her heart, that it was not her sister that she had sent away. She told herself that she had not banished her sister. Whatever it was that terrorized Equestria was not the sister she loved anymore. It who had vowed to shroud the land in eternal darkness, it who felt no love, compassion, care, it who brought the potential destruction of the ponies of Equestria; this could never had been her caring, warm sister.
The princess was so obsessed in her glory and shone as the sun, blindly. She cast a shadow long, and the shadow fell dark upon her younger sister. Years passed, and soon did the princess turn from her light to gaze down on her sister, cowering in pain and darkness. The princess realized with a dread that neither citizens of her land nor she had given the sister the love she deserved.
This is the tale of the events that transpired.

Cover art by Zedrin on DeviantArt