Sometimes it's the negative advice that motivates us to succeed. Mignon Fogarty aka Grammar Girl is a wonderful person in addition to being a great source of useful grammar tips. She shares her story at: Advice from Mignon Fogarty Writing advice from leading authors Gotham Writers' Workshop.
For me it was an English teacher in high school. We had a "local color" writing assignment. When she passed out the graded scenes, she raved about one. Then she couldn't remember who wrote it. She asked for the writer to hold up her hand. When I held up my hand, she said, "You didn't write that," or words to that effect. She turned me off creative writing for about 20 years as a result. Now, she's going to get a mention in the upcoming book - Paths Less Traveled with a reference "Yes, I did write it."
Ok, so Mignon is a better person than I am. I can live with that.