"Try to describe it?"
"If you can."
Sunset Shimmer looked askance, rapping her fingers on the arm of her chair thoughtfully. Her eyes went hazy as they often did, like she was zoning out entirely. When she opened her mouth to speak again, she didn't zone back in.
"We have two eyes, right? And each eye is a camera. So when we're seeing the world, we're seeing two images overlapped, and our brains mush them together and fill it all in. But if we stop paying attention, the two images drift apart. We see double, right?"
"Of course." Her psychiatrist nodded.
"It's like that." Sunset explained. "Except I'm seeing double all the time. And the two images are completely different. And I can't shut it out."
Jot jot jot. Always jotting down notes, she was. A lot to say about Sunset. A lot Sunset could say about herself, honestly.
"May I ask an honest question?" Her psychiatrist posited.
"Shoot." Sunset leaned back in her chair, swiveling her head around unsurely.
"You say that, before you walked through the mirror, there was a human Sunset and a pony Sunset."
"Correct."
"And now there's only you, the pony Sunset. So, what happened to the human?"
Sunset smiled a little.
"You're looking at her, ma'am."
When Sunset Shimmer was taken into hospital care, they could find no family to call. No files on her, no medical history, not even a birth certificate. It was as if she had started existing yesterday.
She had been forcibly taken in after reports of her began circulating, a teenage girl running screaming on the lawn of a local high school, yelling and fighting off nothing before collapsing to the ground into a fit of sobs. The first attempts to contact and comfort her from the concerned locals were shaken off, and she would resume punching and banging on the statue on the school's front lawn (a rearing horse, kicking at the sky) until her knuckles were bloody and raw.
Medical care was called, and she was forcibly hospitalized.
Not long after, she was put into psychiatric care.
She had been there ever since.
When Sunset Shimmer was given her home, she had been found in the middle of the woods, running screaming through the trees and making it impossible for anyone more than two towns away to not hear her. Nobody who saw her recognized her, and when asked where she came from, she froze up and began crying every time. Her body was covered in cuts and scars, although from running through bramble and trees or fighting off hungry animals it wasn't apparent.
The town she ended up in, so far from Canterlot that most ponies were only dimly aware of the princesses, did its best to shelter her, but there was only so much they could do for a wayward, clearly unstable teenage girl. Her magic would flare wildly until someone finally got the idea to stifle it with a tennis ball. She would constantly talk to nowhere and noone, and when she finally calmed down enough to answer the townspony's questions, they were always deliriously esoteric.
"Where do you come from?"
"...S-Somewhere."
"Do you have any family?"
"No."
"What happened to you?"
"I-I messed up. I tried to use- I tried to use a... a thing. I messed up, and now I'm in two..."
Sunset began hyperventilating her first time, and needed to be calmed down by the attending doctor.
"...I'm in two places at once. I-It hurts. It hurts really, really bad."
Night time was Sunset's favorite time. At night, when she was allowed to return to her bedroom, she could neglect Earth entirely and focus solely on her actions in Equestria. She made sure all of her shifts were at night, the time most ponies hated working. In Equestria, she would walk freely through the farm, fixing up what chores she could offer for her middling pay. On Earth, she would walk into the walls over and over again, bruising herself all over and trying to ignore it so she could focus on her pony body.
Sleeping in either body was hard for her, so she tended to wait until she was too exhausted to stay up any more, when one body would shut down and collapse. For a while every morning, she'd lock every door of her house in Equestria so she could go about the institution on Earth without fear of her pony body wandering off into the woods. On lucky days, her pony body slept there. Most days weren't that lucky.
Try as she might, she couldn't explain her need to be locked up in her bedroom more often to her psychiatrists. They saw it as retreating, regressing, a refusal to get better. Nobody believed Sunset about Equestria, obviously.
Eventually, Sunset gave up on trying.
Sunset never told the ponies she shared a town with about being Celestia's protege because she was afraid.
Celestia had warned her. The Mirror Portal was a prototype. She had been given every clear indication of the risks, and she had done it anyway. To face Celestia after this, after what she had become, terrified Sunset.
The humans weren't led by a Celestia. They had a prince or something, someone who could only be prince for a few years, and some smaller leaders Sunset could never remember. Their entire society was patriarchal, a stark contrast to Equestria's mainly female population.
At least, she had heard it was patriarchal. It wasn't like she got to see much of Earth's society from inside her walls.
The mirror portal on Earth hadn't allowed her to cross through on either side. She had tried her damndest, only to be forced to retreat on both sides when she heard the royal guards approaching the commotion in Equestria. She had her suspicions for why this may be, but whatever hypotheses she could come up with were meaningless; She wasn't even in the same city as the damn statue anymore.
Why bother trying to go back home, anyway? She was home.
One morning, Sunset woke up, and found she was cured.
For a minute straight, she saw only her bedroom in Equestria. She was only a pony, and when she looked down she saw only hooves. And it was the most beautiful sight she had ever seen.
For one moment.
Then her human body woke up.
Day by day, it began to get worse, and Sunset was quickly found to be practically inconsolable.
Her pony body wasn't just waking up earlier; Time was moving faster. Little by little, bit by bit, she would blink and the pony in front of her on Equestria was waiting for her to answer a question. Another blink, and it was lunchtime. Another blink, and sleep.
And then just humanity.
And then seeing double.
The first time she experienced three Equestrian days in the span of one Earth day, she had to be forcibly kept in her room under lock and key.
That was fine with her. It made it easier, even as she ran into her walls on Earth, to leave the town she had settled into in Equestria.
She needed to get back to the castle. It didn't matter if anyone needed her anywhere; She needed Celestia. Celestia would fix this.
But as she walked, for days and days and days without the sun ever dipping below the horizon on Earth, she realized her destination was getting further and further off.
Because she was moving slower, slower, slower.
20s. 30s. 50s. She could barely keep track of it anymore.
The orderlies let Sunset out of her room and tried to console her; She was beyond dismal by this point, having cried all her tears. When prodded for questions, she only muttered her destination, walking into the walls of her therapist's office with a purpose.
"Canterlot Castle. I have to get to Canterlot."
God help whoever would try to move her in the opposite direction of her walk. She would scream and yell and fight and bite, watching her precious progress shrink further away by the step until she was returned to her room.
And then it was back to walking.
Sunset only slept out of exhaustion.
On her last day in Equestria, Sunset was both a teenager and many decades her own elder.
Curled up against a rock, still miles and miles away from her destination, the elderly mare looked up into the rain.
She saw Luna's moon overhead, shining down on her.
Her golden hairs had faded, in the places where they weren't silver. Her hooves were worn down and chipped all over.
For 70 years straight, Sunset had been walking.
Or, in Earth terms, a handful of months.
Sunset's Equestrian body thought about how beautiful the moon was here.
Earth couldn't hope to compare.
Then she closed her eyes, and breathed her last.
And Sunset saw through one pair of eyes.
Only her human body remained.
It was a strange experience, being a teenager who had already lived an entire life and died.
When she tried to cry, she realized she was out of tears.
So, she simply waited for the orderlies to get her.
The human named Sunset Shimmer was cured.
