I can relate, ASNerd. I wrote my own stories from the moment I could write. I even had several notebooks with ideas because I had far more ideas for stories than I had time to write. Anything could spark a story for me. I always loved writing stories for school (stories that is, not non-fiction, which I found boring to write (and often read too).). I was always told I was imaginative and creative, and I always got positive feedback on my stories.
Then I turned 20, and it was like a switch was pulled, because I lost my drive to write entirely. For the last 20 years my only writing has been shopping lists, online posts and diary entries. I have only written a handful of stories a decade ago because I had to write them for school. Writing and thinking up stories (often based on daydreams) was so much part of who I was as a kid and teen. I always had something to write about and I always wanted to and felt like it, even needed it. I can still manage to come up with stories from time to time, but I have never found that spark, that strong drive, that I used to have. And I don't understand why it happened so quickly. It wasn't a steady decline, it was close to over night. I didn't lose the interest, I lost the drive.