Ages ago, after the first Harry Potter film, I saw the DVD on sale, decided to buy it (thinking I’d give it away as a gift), watched it, and became hooked. Read the books (1-4 at the time), saw the second film, and didn’t really want to wait. I started to stumble into fanfiction, started reading, (initially I thought it was stolen material, as in, hacking JKR’s computer, before I came to understand it). I enjoyed it and kept reading. There was one particular fanfic I wanted more of, but the author wasn’t updating and I grew impatient. Until this point, I hadn’t written anything since my high-school/middle-school days, so I wasn’t very confident, but I wanted it to continue; I picked up the keyboard, started typing. That first fanfic is still posted (not on AFF because I wasn’t aware of AFF at the time, and it also violates the rules here), the sequel is still a WIP (last updated ~5 years ago); and I had even written adult “versions” for chapters of both fics that I’ve never posted. Nowadays, I write original science fiction in a distorted dystopia that might come to pass.
I suppose the moral is whatever cracks the ice, gets you to write. For me, it was a nearly abandoned fanfic that I felt the urge to continue, and in the process, I overcame my self doubts. My original stories wouldn’t have been written if it weren’t for breaking the ice a decade and a half ago, playing around in JKR’s sandbox for a while.