Princess Twilight Sparkle sat in her throne room, looking up at the magical memory trinkets that had been installed in the ceiling. The roots hadn't been cleaned well enough yet, so fresh clumps of dirt occasionally fell onto the Cutie Map. So many wonderful memories danced above her head. In the years to come, Twilight would look at them again and again, and each time they would remind her of the indelible past that she and her friends shared. It really was the perfect gift to make her castle into a home.
"What a charmed life I have... so perfect in every way," she whispered to herself. Twilight tried her best to smile, but it wouldn't come. The day her friends had surprised her with this beautiful gift had been the best day of her life, hooves down. How ironic that the same evening, this evening, could turn out to be so unspeakable. Twilight stood up and walked reluctantly out of the throne room. She had work to do.
Slowly, the alicorn glided down the long hallway to her bedroom. The magic of her transformation had changed many things about her, even the way she walked. Twilight Sparkle bore more than a passing resemblance to her former mentor, but appearances were superficial. Princess Celestia had grown comfortable living on a pedestal without any close friends other than her sister and advisers. Twilight was nothing like that. She had been without friends for most of her life, and now that her eyes had been opened she couldn't go back. She needed her friends. It was a primal need, rooted deeper than the need for grass or water or even air.
In the morning she would begin work on her medical research. She had carefully memorized all of the formulas she would need to keep Rarity healthy for at least fifteen years, and additional research would extend her life even further. Of course, Rarity and her other friends would still die someday; but the longer they lived, the longer it would be before she'd have to run the risk of abandoning them once again.
Oh, that terrible risk! It frustrated Twilight that she didn't know what happened when she broke the symmetry of time. There were two possibilities she considered plausible. In the less likely of the two, the future would be rewritten entirely, and her dearest friends would cease to exist. If that were the case, putting them to sleep might spare them from the experience, and perhaps even preserve a spark of their souls between timelines. But the more likely alternative was that Twilight would leave their universe for a new timeline, selfishly abandoning them forever. Of course she didn't like that idea, but her friends were stronger than she was. They could live without Twilight Sparkle, even if she couldn't live without them.
Twilight opened her bedroom door and stepped inside. She walked up to her bed and knelt beside it, looking down at the helpless little pony tucked beneath the covers, the pony she knew would be here tonight. There was a loud snoring sound, a sound Twilight hated because it felt so unfamiliar to her. It made the sleeping pony seem alien, and that in turn made the experience more memorable. Twilight didn't want to remember this moment. This is the last point in her life that she would ever want to revisit.
And yet, here she was again; because it was her life to live, and it was the life she needed. It simply wasn't enough for Twilight to be immortal. Her friends had to be immortal too, so she could live with them and share with them and love with them over and over again. Just like all those little ornaments in the throne room, her friends were precious to her beyond words. They were meant to be cherished and remembered and relived, forever. But immortality wasn't possible for normal ponies, so this was the only way to eternity. This was how it had to be, she thought to herself, steeling her conviction for the necessary task at hoof.
"I'm sorry," Twilight Sparkle whispered, brushing aside a tear as she once again cast the spell that stopped her beating heart.