Writing fanfiction is like borrowing someone else’s stage and props.
I’m saying something obvious.
But still, I think it is the biggest reason for why it is so much easier then writing your own thing.

Because, when you write a fanfic, it feels like you can stand in the world that already exists on its own and just describe what you see. You may of course move the figures-actors in your own way, or add the new ones, or mix the world with something else. But still, whatever you do, it plays out on the already existing stage, with engine well oiled and running.
Whereas, when you write your own, it feels like something in between writing a programming code or building a machine from 0 without a manual… you try, and change it, and re-arrange it, and add piece by piece, hoping that one day it will move… because it needs to work, before you can actually stand comfortably and see your story play itself out in it.

Maybe it’s easier if you chose to base it in your everyday world

For me this is the hardest thing, to make what I see work when put in words.