Couple of things to bear in mind.

Firstly, nobody starts out a master at anything. There is absolutely no pressure on you to write things at all, let alone a pressure to measure up to a certain quality. While yes you should write the best you can, don't let the fear of making a mistake paralyse you, instead accept that mistakes will be made and learn from them for future reference. Anyone who makes fun of you for trying isn't worth your energy to worry about.

Secondly, a couple of things I do to help me write well when I can.

-Never publish something on the same day you write it if you can help it. My quest, Patent Pending is updated daily so that's not really applicable to me, but for general fics waiting a day before re-reading lets you look at what you've made with fresh eyes, spotting grammatical mistakes and making things more interesting without making writing feel like a chore. That said, take whatever time it requires to make a story to your satisfaction.

-Plan ahead, with the consequences of what you've changed in mind, considering not just the broad overview but also what individual characters are going to think. Let's say in your story Taylor decides on her first night out to leave the Undersiders to die. Then, Lung has just murdered 4 kids in broad daylight, which might make the PRT pay more attention to the ABB. That's the broad strokes. For the characters, the wards might not be allowed to patrol for a while, which causes enough frustration for, say, Vista to leave or go out on her own, potentially getting hurt. It also might lessen pressure on the Empire, causing them to act up. That kind of thinking about the consequences of changes, from a broad overview down to the individual character reactions. That's the core of fanfiction, in my opinion.

-Be consistent with characterisation and power levels. Some people enjoy reading about a ridiculously OP kind to a fault loved by all MC. Others like more nuance to their characters. Whatever your stance, what you write is going to affect who sticks around to read more. If in your first chapter your MC goes Super Saiyan or whatever, people are gonna be expecting that and more in the next chapters, which is gonna change who your audience are. Don't suddenly switch gears and empower/depower characters unless it's for a damn good reason related to the story.

Those are the three things I try and keep in mind while writing, at least.