How do I know which stories are truly mine and which just sell?

You finally have the authority to make what you want, and the strange fear that came with it is that you no longer know what you want. Years of reading the market can drown out the quieter voice that first made you want to tell stories. That voice is not gone. It has just been talked over for a long time. The way back is not to guess what sells or to guess what is pure. It is to notice what you cannot stop thinking about when no one is watching. The story that nags at you, the image you return to for no clear reason, that is the thread that is yours. What sells and what is yours are not always enemies, but only one of them will still matter to you in ten years. Give yourself one small, low-stakes project with no audience in mind, a scene, a short, a page. Make it only to answer the question of what you would do if no one were buying. The answer will not be loud, but it will be honest, and it will point you home.