Late to this party, but I cannot believe that not one person mentioned that there is actually a word for this!
You are NOT alone - you are a “pantster” - as in “flying by the seat of your pants,” without a plot, outline or other pre-planning (the opposite of a “planner”).
I am also a pantster, although I write non-fiction. A lot of my writing is quite structured and formal, and needs planning. But… my favorite parts to write are the narratives which can go most anywhere, depending upon the audience for that particular piece. I started with lists of topics and ideas I wanted to write about.
It’s common for me to start off writing in one direction, but the story compels me to go in a completely different arc. I let the story lead the way. (and re-file the parts I left out back into the idea file for later consideration)