Celestia found herself alone. The wind was blowing hard around her, and not a pony was in sight. Without her powers, without a way to defend herself, still drained from having saved Luna's life. For the first time in a long time, she was afraid. She feared for her life. She'd forgotten what it was like. She'd forgotten fear of death wasn't about one's self, but about the ones around them. She'd forgotten she could care that way.
While Nightmare Moon had been torturing her, she hadn't been afraid. In a way, she'd felt like she deserved what she was going through. And though she could have died there, she hadn't worried about it. If it happened it happened, so she'd thought. It was for a good cause, and she'd closed out her loose ends sufficiently. That had been then. Things were different. What before would have felt right had come to feel wrong. It had come to feel unfair. She didn't want not to see Luna again. Not after what had happened. Dying to save her she would have accepted, but not that. Not after she'd already succeeded.
She began to walk, shivering in the cold, stepping shakily over the ground as darkness filled it like blood leaking over stone. She dared not fly, she did not believe herself capable of dealing with those winds in her state. She was scared. At every step she looked around, hoping to see a friend, terrified of the possibility of seeing something else. She was alone, and there was no point in lying to herself. No point in hiding her worry. She'd done it so long she'd forgotten how to even feel worried, for a time.
She was needed elsewhere. Ponies could be hurt and she was needed to heal them. She could be the only one capable of saving them. She had purpose again. Something she hadn't felt for a long time. Something she'd missed more than she'd realised. Was it her atonement? To serve and heal others for the rest of her life, no longer a queen or a warrior? She wouldn't have been against it. She would have embraced it. To have something she could do to truly help others. Not make up for her mistakes, no. There was no changing the past anymore, no going back. But she could justify her existence. Let her life be something that brought good, something worth keeping.
Maybe she'd let Twilight keep her powers. No. Maybe a part of them. As her situation reminded her, she could not go without magic of her own. If she was to protect others, she needed to be able to protect herself too. If her life was justified in continuing, it needed support to avoid losing the benefits she brought. Not for herself, but for the good others could get from her living.
Life as a tool, then. Not something she objected to. She'd lived life as it was meant many times over what many others might have. Too many. Too long. If she kept living just to be of use for others, it would be still better than to do so for no reason at all. But to do so, she first had to survive.