After Starlight Glimmer's multiple uses of non-consensual mind control, combined with her volatile mood swings and preference for emotional manipulation to solve simple friendship problems, the entire town of Ponyville raised complaints to Princess Twilight about Starlight's supposed desire to "reform". There was also the issue of whether a former cult leader who nearly destroyed the entire time-space continuum over a foalhood grudge should even be allowed to walk freely. Upon seeing the chaos Starlight was leaving in her wake, and seeing each of her friends becoming warier of her toxic behavior, Twilight confronted her protege about the matter. But when the ex-villain continually dismissed her actions as nothing more than a trivial matter, Twilight realized the full magnitude of letting Starlight off with little more than a slap on the hoof. She had no choice but to do what she should've done from the very start.

"Twilight, what are you doing?!" Starlight thrashed in the orderlies' grasp as they strapped her into the straitjacket. A magic inhibitor was already fastened around her horn.
The Princess of Friendship merely avoided Starlight's pleading gaze. "I'm sorry, Starlight. We should never have shielded you from the consequences of your actions."
"But I've learned so much since then!" Starlight winced as the straps of the straitjacket chafed against her forelimbs.
"Apparently not if you keep relapsing into the same behavioral patterns that led to you starting Our Town. Ponies are terrified of you, Starlight. And they're going to keep being so, unless you show them that you want to distance yourself from the mare you used to be." Twilight shook her head. "And sadly, all you've given us are empty promises."
"But-But what about all those friendship lessons you kept pushing onto me?!" Starlight challenged.
"I didn't expect you to resort to mind-controlling my friends and blasting Discord because of a disagreement!"
Starlight shrunk back as Twilight rose her voice. Even the orderlies restraining Starlight became slightly unnerved by the young Princess' outburst. Twilight took a minute to compose herself before she addressed Starlight again.
"Starlight… This can't continue anymore. Actions speak louder than words, and your actions up until now have shown me that you need help. Help that I can't give you." She then nodded to the orderlies. "But maybe this place can be a good start."
"Twilight…no, please-mmmph!!" Starlight's protests were cut off as a thick leather gag was secured onto her muzzle.
Two of the orderlies dragged Starlight by the straps of her straitjacket across the linoleum floor, as Twilight gradually shrunk in the distance.
"Twlghht! Plshh!! I shrr I'll chhnge!! TWLLGGHHTT!!!" Starlight shook her head fervently as she was dragged into what looked like a small padded cell. Her muffled screams were silenced when the metal door slammed from behind her.

Hours had passed since Starlight was committed to the mental ward a few miles outside of Ponyville. None of the orderlies would let her out, no matter how much she screamed and begged behind her gag. She paced around in the confines of her cell, kicking at the padded walls until she collapsed onto the floor. Starlight squeezed her eyes shut as tears trickled down her face. How could Twilight do this to her? She was only doing what was instructed of her: it was her friends' fault for testing her patience and stalling her friendship lessons. Her sadness and despair quickly gave way to red-hot rage. She should never have accepted Twilight's "friendship offer" if this is what the outcome would be. She should've done more to break her and her friends into staying in her perfect town when she had the chance.
Friend. That word infuriated Starlight the more it echoed in her mind like a taunt. Real friends didn't drop you out of the blue when you're suddenly inconvenient. Real friends didn't shun you and treat you like a freak just for advocating for yourself. Real friends didn't-
Didn't treat you like a puppet for the sake of furthering their goals. Real friends didn't sell your property without your consent. Real friends didn't attack you with magic out of simple heckling. Real friends didn't treat you like a canvas to show off their magical prowess, ignoring your own feelings all the while.
The realization hit Starlight like a wrecking ball. Did I do anything right??
Starlight erupted into a fit of muffled sobbing when she realized the answer to her own question. The faces of Sunburst, Party Favor, Trixie, Twilight, and so many other ponies flashed through her mind in an instant. So many ponies that she hurt, used, manipulated, and conveniently forgot about when they weren't useful anymore. Just like all the bullies in her fillyhood years. Wasn't the whole point of Our Town to be a safe haven from ponies like that? Was that just another lie she told herself?
She glanced up at the dark ceiling of her cell through misty eyes. I'm gonna do it right this time. I want to change. I want to be better!
But here she was, all alone in a little room where nopony wanted to see her. And it was all her own fault. Starlight blinked when she remembered exactly where she was. This wasn't prison, just a mental hospital where she would get treatment. She would admit that she let her emotions get the better of her sometimes…okay, maybe a lot. Especially if she kept blowing up at every little inconvenience around her. Especially if her own friends (were they even her friends at this point?) walked on eggshells around her. Starlight smiled grimly when she realized it was Twilight who was probably paying for her treatment. The fact that she was even doing so for her despite everything she did gave Starlight a new sense of direction.
I can make this work. I'll be good. I'll be the best patient these doctors ever had, and then if I'm let out, I'll change. Really change. No more brainwashing, no more unprovoked arguments, no more dismissing other ponies' feelings. I want to be good. I swear to Celestia, Luna, and Faust above, I'll be a better pony.
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—Starlight is a character that I don't particularly like. I'm not a fan of how the writers constantly give her a free pass for all of her bad acts throughout the later seasons of MLP without any consequences. I know one of the show's core themes is that everyone deserves a second chance. But the writers missed the part about how people actually have to work towards changing themselves and righting their wrongs, not just give a sob story and expect a free pass.
—On the other hand, Starlight is essentially a walking disaster in canon and it's fun seeing characters like that in predicaments like this because of their own screwups. I was veering towards Starlight swearing revenge on the Mane 6 again, but I hoped to challenge myself by actually having her self-reflect during her time in solitary.
—I'm sorry if the short story feels a bit rushed, but it's late as I'm writing this and I wanted to get this out before bed.