Alone
The following day, after spending most of the day in the ancient castle, the two royal sisters were ready to start their new day. "Well," Princess Celestia said, "I must be off." "I await your return, sister," Princess Luna said.
Actually, it'll be mostly Princess Celestia being active for the day. On that particular day, Princess Celestia left Canterlot to visit Ponyville on one of her special occasions, mostly to visit her best student, Twilight Sparkle.
Princess Luna, of course, stayed behind in the castle. "That was a fun day," Princess Luna thought to herself. Princess Luna felt happy after visiting her old place.
"Thank you, sister," Princess Luna said, "That was a nice thing you've done for me in a thousand years." Princess Luna, "Thanks sister. I only wish I could do better myself." Princess Luna's next sentence made sense.
"I really shouldn't have done that to do a thousand years in the first place," Princess Luna said, "It is one of my biggest regrets now. I shouldn't have let my jealousy get the better of me. But that's in the past now. I'm free from my own darkness! I can do this on my own!"
Soon, Princess Luna set hoof outside. "I wonder if I could think out here," she thought to herself, "Maybe I could give it a try." So she did. She stood still and thought about what she is going to do for the rest of her life.
"Should I keep living in the past and feeling guilty of myself?" she asked, "Or should I move forward with everypony?" She thought about this for a while now, but she had made her final decision.
"I choose to go forward with my life and never to try and plunge Equestria into permanent darkness! That is my final decision! And I am proud to make my choice. It is my destiny!" She didn't say it out loud. She still didn't want any other pony to hear her at this time.
"Maybe when I'm ready," Princess Luna said, "And when I'm back to full strength. And, of course when my sister returns." So Princess Luna went back inside.
When she returned to her room, Princess Luna has decided that she will try to do her best in the future. But deep down inside, she really isn't 100% sure about that. "I'd better decide for myself," Princess Luna thought. She remained in her room for the rest of the day.