But I find myself developing this story as I write.
When I first started out writing in 2015, I made the same mistake. I didn’t think it was an issue at first but when you go deeper into a character’s motivations for his/her actions, that’s when the issue is profound. If you don’t know where you’re story is going before you start it, you won’t be able to grasp the true traits, personality and characteristics of your characters.
Plot points, subplots, flashbacks – even if they don’t entirely make it to the book they are very important for your own understanding of the situation and has to be related in some way to the readers, even if just in passing.