My stories usually stem from an idea.
The fic I'm writing now (as in, it's a fanfic) stems from a popular fan theory for a game I play, but I took the theory and said "what if it were formatted like" a certain TV show. It would make more sense than how the game presents the story.
As far as depression around writing goes... there are way more people than ever writing now. Therefore, fewer people than ever, especially percentage-wise, are being paid well for it. Plus AI. Write for yourself. I'm writing a fanfic, so I know I'll never sell it.