Here's a pre-canon one shot in honor of Shelly's upcoming November 24th birthday (as confirmed by the show's social media). Not based on any prompts.
Shelly had woken up in a good mood for once.
Her tenth birthday yesterday was probably the best one that she had ever had. Thanksgiving had fallen on the 22nd this year, so for once, the holiday didn't overshadow her big day. Her mother had taken her and some friends to see the new Sadie Dakota movie, followed by cake and presents. Shelly thought that she had gotten a pretty good haul this year.
So far, being in double digits was pretty great.
Shelly joined her family at the kitchen table and poured herself a bowl of the Mermaid Madness cereal that she had begged her dad to pick up at the grocery store.
Sharon shot a disapproving look at her daughter's breakfast. "Really, Shelly. That cereal is pure sugar! How about some oatmeal instead?"
"No, Mom! This cereal is going to be great!" Shelly spooned some of the pastel puffs into her mouth.
She nearly gagged.
Something was wrong.
The commercial had made the cereal look like manna from heaven. And the pink and aqua shell and starfish-shaped puffs were so pretty.
The cereal sure didn't taste pretty. In fact, it tasted like literal shit. Or at least what Shelly imagined shit to taste like.
Shelly did a double take. Her cereal was no longer pink and aqua, but an ugly shade of brown. The cute mermaid-themed shapes looked almost like… turds?
"Is something the matter, Shelly?" Sharon asked, noticing the disgusted look on her daughter's face.
"I think that the cereal went bad," Shelly managed to choke out.
"That's not possible. It doesn't expire for another six months." Sharon inspected the garish cereal box.
"Maybe it's the milk!"
"The milk is perfectly fine, Shelly. Maybe the cereal just isn't as good as you thought it would be."
Shelly wasn't about to prove her mother right. "I'm telling you, something's wrong with it! Here, turd! Taste this!" Shelly thrust her bowl towards Stan.
The six-year old cautiously took a bite. "I like it. Can I have a bowl, Mom?"
Shelly rolled her eyes. What did her kindergarten baby of a brother know anyways?
Shelly slid the Sadie Dakota movie soundtrack album into her CD player. Yesterday's movie had been one of the best she had ever seen and she wanted to forget about her nauseating breakfast by losing herself in the music.
Something was wrong.
The noise that blared from the speakers wasn't the fun, bouncy bop that she remembered. It sounded suspiciously like her dad after a Taco Bell binge.
Maybe it was some kind of joke. Shelly skipped to the next track. And the next. No, it was all the same.
Did they put the wrong CD in the case? Shelly wondered. She took the disk out of the player and inspected it.
No, it was definitely Sadie Dakota. Shelly sighed and put the CD back in its case.
For the second time today, Shelly did a double take.
The blonde, bubbly pop star no longer adorned the album cover. She was replaced by what looked like a giant turd.
A confused Shelly flopped onto her bed. That cereal must have gotten to her.
She didn't know what was happening to her.
It seemed like everything she had once loved was now shitty. Her food, her music, her favorite TV shows. Even her prized Zombie Junior High doll collection looked more like it belonged in the toilet instead of proudly displayed on her shelf.
Shelly was miserable and confused. And when Shelly was miserable and confused, she got angry.
Her latest bout of rage had landed her in the principal's office for the fifth time in a single week.
"Your daughter's behavior has been out of control lately, Mr. and Mrs. Marsh! I'm used to seeing her in my office for fighting, but not every single day of the week!"
"I don't know what to do, Principal Victoria!" Sharon despaired. "Shelly has always had a temper, but I've never seen anything like this!"
"Is everything alright at home?" Mr. Mackey chimed in. "Sometimes these things manifest at home, mmkay?"
"Everything's fine," Randy replied. "Except that Sharon's dinners have been a bit… lacking these days."
"Go to hell, Randy! Maybe if you picked up a bit of the slack once in awhile, I wouldn't be so overworked!"
Shelly tuned her parents out as they continued to argue.
Shelly sat glumly on the couch. Everything was even shittier than usual. She doubted that she would ever be happy again.
Tears sprang to Shelly's eyes. She blinked them away. She was ten now. Crying was for babies. She would just have to find another way to cope.
Shelly stared out the window. Her brother and his green hat turd friend were running around the front yard playing some stupid game. Their laughter was like a needle piercing her eardrum.
She couldn't stomach the scene any longer. She had to do something. Shelly clenched her fists and headed for the front door.
If I can't be happy, then he doesn't deserve to be happy either!
It baffles me that almost nobody in the fandom has latched onto the idea that Shelly also suffers from "cynicism". It makes so much sense. It's also very common for depression to manifest itself as anger and irritability.