Why my friends should stop asking me to write stories for them

I'm actually willing to help people out with writing stories especially if they're people I'm really close to and they're running a deadline. I'm not the best, I know, but I try to help them as much as I can.

A while ago my sister asked me to help her write a story for an English project. She had to make a children's book from scratch. She had to write a story by herself and draw illustrations, print out page after page and bind it together. Unfortunately, she had several projects waiting and was sick so I agreed to help her. I wrote the story and made the illustrations.

I named the protagonist - Phoebe Elizabeth Fell - after a friend of mine. That's not their full name, of course, I just asked to use one of their names and they agreed. The vampire's name just popped into my head the second I wrote him and I stuck with that.

It was supposed to be a short, short story. Maybe a thousand words, or a thousand and a half. Unfortunately I got too into it and wrote it up to 4k or so. I got so invested in such a small amount of time I classpected the characters, including Phoebe's brother who didn't even have that big a role in the story. I got too invested in the designs. I ran so many scenarios through my head and just got so into it.

Another time a friend asked me to write for them was when they had to make a mock magazine, and there had to be a part where they had to include a story. A short story. And they gave me free reign and at that time, I was just thinking of a way on how to turn the soulmate trope from something sweet to something horrifying and life-threatening.

So I wrote a story about soulmates, and had all these rules of this world figured out. I had systems and traditions and beliefs created; I got too into it the same way I got too into the kid and vampire story. And the way I get invested is usually to classpect, assign everyone their moons and their patron trolls, and everything else. Instead of a short story, my friend ended up with a 24k story about two assholes working out a soulmate pairing so they both don't die.

I had my classmates read the story because it was going to actually be read and graded when my friend included it in their mock magazine and everyone got way too into it too. They passed the copy around - it was the copy that still had red notes on it because I was proofreading it - and word about it just started to spread to the point where I didn't even have to ask other people to read it because they just asked to read it since they heard it was funny. It was flattering.

Now I'm currently trying to write the...5th? 6th? draft on the novel for it. The story has expanded a lot and I'm actually trying to explore the side characters better and just hint at all the rules and systems of this world. 

I love writing stories and I just get way too into things that short stories just turn into sprawling novels. And the worst part is that I try to write around twenty ideas all at once, and everything is just a huge mess.

It's even worse if my friends actually need me to write a short story. What they get will not be short.