When writing stories, whether they be short or over 200K words- organization as the key between a level head, and having a mental break down because you can't remember the name of the town dog that was a hundred pages ago.
I am a partially off the cuff writer, so I'm not saying you need to make outlines for your stories. I don't 90% of the time, but I am saying, keep notes of all the things you decide to just write into your story. So you don't have to go back to the first chapters every few pages because you forgot what quote you used on the hilt of your MC's blade.
These are just something I learned over the years trying to balance several stories at once. It isn't super magical, but it helps keep my stories organized enough where the papers are mixed together.
FIRST. I suggest writing everything down- yes, writing it down. Have a paper copy of these things in case your computer dies, or goes down, you don't lose EVERYTHING. I also suggest buying a Drive to put your writing on, just in case something happens to your computer and it dies (which has happened to me), at least you have a somewhat recent copy of your writing on the drive.
~ Get a Binder
You can get on at the dollar store or recycle one you already have, but they are SO helpful. If you have separators that can help by keeping things separate; plot, characters, locations, items, languages, etc. I have three books in one series and having a binder lets me keep all three books in one place without our having to go look for all of them separately.
~ File cabinets
I have a filing cabinet and I use it to separate my stories as well as my files for short stories, long stories, fanfics, etc. Especially if you have a lot of stories in which you take breaks from, it keeps you from losing the papers and makes it easy to go back to the ones you have the time or energy. If you don't have a filing cabinet that's okay, they actually have filing totes you can buy that do the same thing.
~ Manilla folders are your best friend.
I used Manilla folders for every story if I don't use a binder. I print things off, have sketches of things from the book, tossing it all in a manilla fold makes it easy to keep together. I tend to have so many things, so normal folders don't work for me; I always rip the pockets. Plus they have the tab at the top you can write the title or genre and find it easier than opening the folder and trying to identify the contents.
~Lay everything out on separate sheets
Don't try to cram everything that falls out of your head onto a single piece of paper, spread it out and make it easy to read. You will be having these sheets for a while during the writing process, you want to be able to go back and actually read them. You draw out a map of a country and then decide you want your characters to live in a specific town; TWO papers. Don't try to squish all the details of the town on the back! I only stress this because it's easy to do, once the ideas start flowing it's hard to stop, let alone look for a new piece of paper. Take the time, you'll be happy you did later.
~Don't be afraid to use a lot of paper
Seriously. Stories are difficult and complex and a lot of plan out, if you have to use 100 sheets, used a hundred sheets. I think I have honestly used that many on one book. Adding outline, research, characters, maps and so on- it's not hard to do.
~If you aren't sure you should write it down; write it down
If you're researching anything for a book and are unsure if it'll be important, write it down anyway. I can't tell you how many times I have done research, read something, thought 'no I won't need that' and then a couple of days later realizing I did need it and trying to find it again online? It's annoying and stressful, it also disrupts the writing flow. Do yourself a favor and write it down. The worst thing is you won't use it, and then it's just extra information for the future.
~Have fun with it
Organization can be fun if you try, some people find it annoyingly tedious, but I like it. Make a cool title, draw pictures of your characters. Highlight and use color pencils, again, you are going to be using this for a long time, might as well make it look nice. It's taking on a whole world of its own and you don't want it to be dull.
Creating a story is hard, but if you keep it organized you can create a more complex and realistic setting. Keeping your characters personalities and flaws organized and written out can help with development and make them better- more rounded characters. It doesn't have to be super compulsive about it, but keeping your papers together and your files put away helps more then anyone can imagine. If you want to put a story down, it's okay, throw it in a folder and come back to it, when you're ready, everything is there and you have no worries.