A workshop mentor of mine recently asked her workshopees, "What was the one thing that made the biggest difference in your writing, whether it was a piece of advice, a teacher, etc.?"
For me, it was hard to pin down. I remember other significant epiphanies; the moment when I realized I could actually read when I was about five and reading the Little Golden Books version of Snow White. Or the time when I finally understood how to read music while practicing flute on the Indiana Jones theme for band. (I made it to first chair after that.)
But...writing? Several things came to mind. The first is actually non-fiction. My tenth grade English teacher asked us to write an essay saying, "I am like a . . ." I decided I was like a crow (don't ask the reasons, either I don't remember or don't want to remember) and wrote about it. A friend read the essay and said it wasn't really an essay because I didn't prove why I was like a crow and went on to explain how to format an essay. That one little conversation made a world of difference when it came to writing high school and college papers and prompted one college teacher to write, "Too bad the English department lost you to music!" Ah, foreshadowing. . .
As far as fiction...I've written since I was little but didn't start seriously writing until after college, but I can't pinpoint one great epiphany.
Maybe it was in grade school, when I was lucky enough to know that creative writing and storytelling was valued because the teachers were kind enough to type out my stories, laminate the covers and bind them with my illustrations.
Maybe it was the initial realization that writing in other people's worlds, fun though it can be, will only take me so far. Writing in the Star Trek universe helped jumpstart my writing, because I didn't have to worry about the world, but could focus on character and story. Once I had the story, I moved it to my own world and began to hone my craft--which resulted in five fantasy novels I had high hopes for but will likely remain in the depths of my hard drive forever.
Maybe it was the online writing workshop which kindly told me info dumps and fantasy names like Eryk'esth'y'a'valen were bad things.
Maybe it was the community college teacher and her short story class where she assured me my story was good, which gave me the courage to write more stories and put them in front of people, which eventually got me into the Clarion SF/F workshop.
Maybe it was Clarion, where I didn't have to be published to feel like a writer, and the friendships I made, which meant connections with more people and other workshops and whole networks of writers whose advice and friendship I continue to value.
Or maybe it's simply knowing I can tell stories people want to read.
What about you? Was there something that really, truly made a difference for you in your writing?