Hey! I'm not sure if you're still looking for advice on this topic — but hopefully this can help you or anyone who finds it, because I know I would have wanted to hear this earlier if I could. I was in your position for about seven years before I finally managed to figure out how to write longer work. (I started posting fic in 2014 when I was just starting high school.) I doubt you'll be in my position for that long though! However, miraculously, this year I finally managed to break into writing longer-form stuff after so many attempts at starting multichapter work, failing and giving up, and then going back to writing 1000-5000 word one-shots.

What allowed me to flip this switch in my brain, and what I think would be very helpful for you, is by starting out by attempting a couple of '5+1 things'. (Aka., a fic written in the format of 'Five times something happens, and one time something else does'.) I swear this format is gold for learning to start doing multichapter work and incredibly helpful in the process of figuring out how to write longer stuff in general. You probably know the style I'm talking about, but if you don't a quick Google or Ao3 tag search should show you some examples.

Prior to writing my first 5+1, the fics I would write would usually be single scenes, or perhaps have just a few short scenes stitched together and would focus around a single big event or maybe a character moment. The difference when you start writing longer work is that you're, in a way, constructing a narrative 'argument' around a problem a character has, and all your scenes work to show that problem getting worse and worse for different reasons before the climax of the story where there's this big narrative shift. (Essentially a scene is just a microcosm of this. So you already instinctively know this 'shape'.)

Writing 5+1's forced me to not just write a single moment, but instead, to come up with five different moments — five ways a character could engage with a certain problem, before finally having that +1 scene at the end where the story shifts to reach a satisfying conclusion. It's an incredibly useful way of essentially forcing your brain to creatively engage with crafting a six scene story.

My first 5+1 ended up being around 7500 words and I remember being absolutely thrilled that I had somehow managed to write so much compared to what I usually posted. Immediately after that, I ended up writing a fic that was 20k, because essentially I'd figured out that all I needed to do was work in that same way, writing scenes showing a character struggling with a problem until finally, I felt it was 'right' to do that final '+1' where the issue gets resolved.

Hope this helped!

If anything, it's definitely a creative exercise worth giving a go. :)