Writing isn’t just for writers. It isn’t just for school, either. Writing is a tool for thinking.
You don’t control your thoughts. They rise from a weird combination of past experiences and held knowledge interacting with the environment. You cannot think about what you’re going to think in advance of thinking about it. Yes, it gets confusing. Our brain is messy. And that’s even before some other idea comes crashing and displaces the previous thought.
Writing is the act of putting thoughts into paper. Once they are in paper, they can’t escape or be displaced, and what’s better, you can start shaping them and refining them.
You can sort out a surprisingly large amount of troubles by just writing them down and working at them on the paper (or screen, but I’m particular to paper), where they can’t escape (and so can’t you).