Celestia visits regularly, up until Twilight is in her fifties or sixties--definitely aging, but still rather young. Celestia notices that she's starting to lose weight, and is developing a bit of a cough, and urges her to go see the doctor. Celestia makes a special visit to see her the day she's supposed to hear the results. She lets herself into the library, and finds Twilight sitting quietly in a window seat, staring out at the landscape. Twilight looks up as Celestia enters--and Celestia finds that she doesn't need to ask her what the doctor said.
Twilight knows her time is short, and her family quickly rallies around her. She tries to teach them what she can, in addition to writing her own memoirs and otherwise getting her affairs in order. She puts on a good face, but secretly admits she's scared: she's not afraid of whatever happens next, but doesn't want to leave her friends and family behind--not when there's still more to do. Flash keeps his own feelings buttoned up tight, but confides in Celestia that he's terrified.
In the meantime, Celestia is doing everything she can to save Twilight. The best doctors in Equestria are at her beck and call, and Celestia has opened the treasury to pay them. More importantly, Celestia tries to visit Twilight and lend support when she can--but, every time she sees her, Celestia can think of nothing but Starswirl's Journal, burning in her fireplace. The image comes to haunt her dreams.
Despite everything, Twilight wastes away far too quickly, and soon she's in the hospital. Celestia does everything she can to force the doctors to make her better--until Twilight herself gently asks Celestia to "Let me go." Celestia, stunned into silence, suddenly turns and leaves.
The next day, Celestia receives a thick packet of letters, postmarked from the hospital. As she examines them, she realizes that they're Friendship Reports, written by every member of Twilight's family, from the youngest granddaughter, up to Flash Sentry himself (who writes a somewhat-stiff, formal note in painfully-precise script).
The very last sheet in the stack was covered in thin, spidery letters, with ink in several different colors—as if the author had to stop and start again several times.
Dear Princess Celestia,
Today, a very good friend of mine gave up something that was very, very dear to her. I’m not sure if she entirely understands how much this means to me; wherever I go next, I hope to somehow repay her for her kindness.
She taught me the power of sacrifice. Giving up something dear to you is the purest form of love—it shows you care for your friends more than you care for yourself. I never really understood that until she showed it to me.
I thank you for all that you’ve done for me. It will never be forgotten—not by me, or by my family. I promise that.Your faithful stu Love,
Twilight Sparkle
At that moment, a palace guard bursts into the room, followed closely by a doctor. Celestia looks up, then begins to cry.
[Excerpt: "Let me go"]
At the funeral, Celestia, wearing her dark dress and low, wide-brimmed hat, is seated in the front row, off to the side. All of Twilight's many friends and relatives are here, all of them touched in some way by her life. Celestia can tell just how much she meant to all of them--and how empty their lives are now that she's gone.
Celestia hears her name spoken, and looks up. Every pony present is looking at her. After a moment of silence, the priest repeats his request: he wants to know if, as her dearest friend, not to mention a Princess, if she has anything she'd like to say.
Celestia swallows. She knows what she would like to say: she'd like to tell everyone about how much of a failure and a fraud she is. She, the immortal princess of the sun, the ruler of Equestria for over a millenium, who has fought demons and dragons, who has challenged literal gods, and who has hammered the lifeless wasteland into a growing green garden fit for all ponies everywhere, cannot even save the one thing she truly cares about.
But she could have, once. Once upon a time, she could have saved Twilight. She held it in her hooves--the spell that would have fixed everything. The spell that would have kept her alive, and kept her from suffering, and prevented all this from happening. But she was too weak, and too scared. She didn't know how badly this would hurt--not just her, but those all around her. If she had known, then perhaps--perhaps she would have--and then, none of them would be here right now. None of them would be sad. And Twilight--Twilight would still be here. And nothing would be able to hurt her anymore.
Celestia opens her mouth to speak--but quickly closes her mouth again and shakes her head. Luna, sitting next to her, notices her distress, and quickly stands, gives a short speech for the two of them, and sits.
Later, at the reception, Celestia is sitting by herself, beating herself up for everything that she could have done, but didn't. All the mistakes she made. All her failures. How much everypony is hurting, all because of her.
In the midst of her reverie, Flash Sentry gently approaches her. He takes a seat beside her, then, without looking at her, begins to talk. At first, he talks about nothing in particular--all the ponies who have made it to the funeral, all the letters and cards and well-wishes they've recieved, and so on--but Celestia does not respond.
Flash glances at her, then sighs heavily. "I just wanted to say," he says finally, "thanks for everything."
Celestia does not react. Flash waits a moment, then continues: he wants to thank her for how much she's done over the years. And, especially, to thank her for how much she's meant. To all of them. Twilight's friendship with Celestia was one of the deepest and closest that she ever had. True, he and Twilght have led simple lives--but those lives have been full, and warm, and loving, and Flash can't think of any way they could have been happier. Their lives wouldn't have been the same without Celestia--none of theirs would.
Flash hesitates, then thanks her again, this time for how much she tried to help after Twilight got sick. She did everything she could, and, though, in the end, she couldn't be saved, it still meant so much to both of them that she'd tried.
Celestia takes a deep, shuddering breath, then looks up at Flash. I could have saved her, she thinks to herself. If I had been a little stronger, then she could have told me that herself.
Flash meets her eye, and, frightened by what he sees, gets up and walks away. Celestia watches him go, then turns her gaze to the floor again.