How do you know what to write?

Sorry if this isn't the right flair- wasn't sure, checked one, started the post, noticed another, checked that one, got distracted for twenty minutes, attention span of a butterfly, whatever. Also, sorry if formatting is weird too, I'm on mobile and it's dumb.

So, I've always struggled with getting the confidence to actually write anything. Usually, the advice is "just do it, it'll be fine". I need to build confidence in my writing by actually writing. And, y'know, that's probably right- but every time I try, I run into this problem.

The moment I get past paragraph 4, I run out of ideas for where I'm actually trying to TAKE the story. It doesn't matter how long it is; oneshot, a couple chapters, an entire book, I can never actually think of where to take the scene. This is mostly about fanfics, but I struggle with my own universes too.

And it's not just scenes, either. Sometimes it's plots. What am I actually planning to DO here?? Where is this idea going? Will it ever come back? I have so many ideas, but none of them are finished, and all of them are in completely random spots in the story (and usually, none of them are the ending. Thats mostly a problem for longer stories though.)

I think I've only ever finished a single piece, and that was me turning a canon scene into writing form, so I already had the direction completely laid out for me. also it's old and i hate it but whatever lmao

So, like. How?? How do you do it? How do you just know where to take a scene, or a plot point, or a story??? Any advice would help, honestly. Do I just need to get over it and write something, and eventually I'll get better at it...?

(Actually, I'll take anything to motivate me to write my stupid stories. I have so many of them please dear GOD get them out of these messy docs and into actual words 💀)

Edit: Thank you to everyone who replied! It's all been super helpful, and I'm hoping I can figure something out that works now. I really appreciate it <3 (also thank god google docs has tabs now because that is also going to help IMMENSELY)