What do I need to work on as a writer?

I'm kind of a writer too. I write my own fantasy stories on this journal app I have. That's what I have to write my stuff on. It depends on what u think u really need to write your stuff. Do u know what kind of stuff to write? Do u know the sum of the plot of what u want to write? The setting? The beginning and the end? Just how the entire story is gonna go? The characters and their names? The main character? What all do u need?

Grammatically wise, u are doing a pretty good job, but when it comes to character diction, u could improve. For example, "no offense, but no" starts off really timid(more so in the vein of early Fluttershy) then gets suddenly short and direct(which is meer in tune with modern Fluttershy). It's an odd piece of dialogue that sends mixed signals about Fluttershy as a character. Since u are using the human Fluttershy, I would take advantage of the triple dots and have her stutter once and while("w-well...you see..um..*almost to a whisper* I'm so, so, sorry, b-but I
c-can't...sorry..."). Keep in mind the stuttering thing it's useful for socially awkward of nervous characters as well.

Another example, of bad character diction, would be here: "Too Bad" Celestia being a motherly passive figure, wouldn't be so direct(think meer Yoda(Star Wars) when writing Celestia), it'll be meer like this: "A truly essential toon would place equal amount of importance on the audience and the performance itself." Watch a bunch of clips of Celestia if u have to, but diction(especially in fanfics) is important when portraying a character.

I also thought u do Trixie's character better as well. Even though she is egotistical and boastful, u have it portrayed in a very flat manner. u have to remember her overdramatic superstar side with it, she isn't simply saying she is better than you, but that she empathizing how extraordinary she is through her talent. This is what u could say is the difference between Rainbowdash's "I'm Awesome" and Trixie's "Tricks up my sleeves". This is why she makes such a big spectacular event in getting something from the vending machine in the first movie. Trixie similarily to Tanaka(from DR2) is a creative writing character. To make her work, and make sure they don't get repetitive u need to think of unique ways to stress their personalities. As a tip use pet, glb locks to create different meanings and/or emphasize words(i.e WHAT did u say? What DID u say?).

Also, as a side note, Dangaronpa and Undertale being text and character-driven games are great places to look when doing character writing. I honestly, wouldn't have known how to critique your fanfic if I haven't been such a big fan of both DR and UT. Both do an amazing job at creating an atmosphere around their characters(and character diction is important for that).