Star really seems to like Earth, so I started wondering what would happen when she had to leave. This is what I came up with. :P
It started when her mother had fallen ill with a rare and deadly disease called Mewnitis: a virus that affected the brain, slowly making it shut down. Only the luckiest victims survived, but her mother wasn't one of them. Naturally, she was devastated, when she heard that her mother had died. A few months later, a hoard of monsters had attacked the village, and her father had died trying to protect the villagers. Now, she was an orphan, and only one thing could have made that worse.
Princess Star Butterfly knew that she would eventually have to leave Earth to become the Queen of Mewni, but she never imagined that it would happen so soon.
She thought about leaving her Dimensional Scissors on Earth, but the Royal Troops would find out and take them back. They would say that it was "for her own safety." They were paranoid. They thought that other dimensions were too dangerous and should only be traveled to as a last resort.
Star loved Earth. She would never admit it to anyone, but she had begun to love Earth more than her home-dimension. Specifically, the people. Sure, she had plenty of friends in Mewni, but she could hardly bare the thought of leaving her Earth friends behind. She would miss them so much, and they would miss her...
And, that wasn't fair. She was the one leaving them. They didn't deserve to hurt. They were all so nice to her, and how did she repay them? By leaving. She deserved to hurt, not them. The realization made her regret trying so hard to make Earth friends. She knew that she would have to leave one day, but she made friends anyway. She was an idiot and a bad friend.
So, she searched through her Magic Instruction Book for a solution. After an hour of reading, she found a spell that would work. She didn't like it very much, but what choice did she have?
Star used the spell on every Earthling she knew, except one. She was saving the best for last. She didn't know if the spell would work, but she didn't want to stick around to find out. Doing so would make her feel even worse.
She packed up all of her Mewnian belongings - the Earth ones would hold too many memories - and magicked her makeshift room away. The Diaz residence was now exactly as it was when she first arrived at the suburban home. A wave of nostalgia coursed through her, and it took everything in her not to start crying.
Now, for the really hard part.
Star walked solemnly down the hall, took a few deep breaths to compose herself, and knocked on the door she had arrived at. "Marco?" Her voice shook, and she cleared her throat. "Can-Can I come in?"
A casual, "Yeah, sure," came from in the room. Star opened the door to find her best Earth friend - no, her best friend - Marco Diaz, wearing his karate attire and preparing to chop a board in half. Star opened her mouth, but Marco raised a silencing hand, his cocoa-brown eyes still glued to the piece of wood in front of him. "Just a sec, Star. I'm in the zone."
Star bit her tongue, allowing her friend a few more moments of happiness before breaking the news. She watched as Marco breathed in and out, in and out, and then let out a hearty battle cry as he slammed his hand into the wood, breaking it in half. His face light up in a disbelieving triumph.
"I did it," he breathed. He then whooped and jumped up in joy. "I finally did it! See that, Star? All those monster fights finally paid off!" Star whimpered as she failed to contain her tears any longer. Why did he have to bring up the times they'd fought monsters together? Marco's delight quickly turned to concern. "Star? Hey, what's wrong?"
"I-I..." Star choked. She swallowed thickly. "M-My dad died, and..." She covered her mouth with one hand in a pathetic attempt to muffle her sobs.
Marco ran a hand through his hair. "Ah, man. I-I'm so sorry, Star." He walked up to her and tried to wrap his arms around her, just like he had when Star's mother passed.
But, Star poked her wand into his chest, stopping him. She shook her head and stared down at her feet. "No, I'm sorry."
"What?" Marco asked, and Star could imagine the confusion on his face. "Why are you sorry?"
Star swallowed hard again. This was for the best, she reminded herself. It was best that he forgot about her, just like everyone else did. She gripped her wand tighter, and saltwater warmed her cheeks as she said a hesitant, "Apple Pie Amnesia."
A puff of neon green glitter erupted from Star's wand and coated Marco, leaving him with a strong scent of fresh apple pie. He went slacked-jawed, his arms dangled limply at his sides, and his eyes took on a glazed, vacant appearance.
Star fought back the rest of her sobs and pulled her Dimensional Scissors out of her purse. She regretfully cut open a portal Mewni, stepped through it, and did not look back. If she looked back, she feared that she would never leave.
She arrived in front of her parents'- In front of her castle, and the portal closed. A fresh wave of sadness crashed into her, and she reminded herself that queens did not cry in public. She wanted to go back. She wanted to go back to Earth and live with the Diaz's and go to Earth school. But, it was too late. When everyone woke up from their trances, they would not remember the name "Star Butterfly." That was how the spell worked. You thought about what you wanted someone to forget, said the incantation, and, poof, that person forgot.
She squeezed her watery eyes shut rubbed her temples. A queen had to concentrate on her own kingdom. Her next action was forced, very much forced, it was for the good of Mewni.
So, before she could change her mind, Prin- Queen Star Butterfly pointed her wand at her forehead, thought about everything she ever knew about Earth, and whispered, "Apple Pie Amnesia."