I have always been a competitive person, in my own way.
Growing up the sort of person who carried the basketball down the court, and tripped on her two left feet on the track, I had to find different ways to be competitive than in the sporting arena. So, I excelled at Debate Club and Model United Nations, but my competition was nothing but boys, and every girl growing up in the 1990s knew that boys don't like girls who schooled them in politics. I did what every boy-crazy teenager did in those dark days, I turned my competition inward: I celebrated everyone's successes, and flogged myself with my own, using them as the new zero.
In essence, I made myself my own worst enemy. Not the best for self-image, but it turned out great for excelling in schooling, professional life, and nerdy ways. Before geek became synonymous with chic, boobs could not sit around the table with young men and tell them how the rules worked without getting told how to do things incorrectly. You just kept your mouth shut and lead by bad-ass example. Which also did not go so well for snagging dates.
One growing up a girl nerd in the 80s and 90s helped me to love is failure, and you get a lot of it as a writer.
I send off my stories, and I've heard nothing but rejections, but I'm starting to hear some really good comments back on the stories I'm sending out. I get the story back, do some edits, turn around and send it to someone else (after moping around the house for 30 minutes, talking about how I'm a hack writer and will never succeed). I'll never get selected if I don't push past the point where I stop being bad at what I'm doing, and I got into this because I love writing.
I set a new bar for my own writing every time I see someone else do it better, no matter who they are. I read what they do, and if I like it, I try to mimic it in my own voice and my own way, trying to make it fit with whatever I'm working on. Irvine Welsh taught me how to write despicable people. Barbara Kingsolver taught me how to write non-cliched similes. George Orwell taught me how to create mood. Terry Pratchett taught me how to punch down while talking up.
I wrote fan-fiction for nineteen years before I decided to start taking writing seriously. It was a magic day when I decided to start working on my novel, my first original piece of fiction. At first I thought 350 pages was insurmountable. The book's now hovering at 500 pages, a behemoth of a thing. Writing fan-fiction for those nineteen years, I found a passion for writing, and wanted to take it somewhere beyond Dungeons & Dragons, World of Warcraft, and Star Wars into something of my own design.
So what if I've made a lot of mistakes in my writing? If I don't keep setting that bar at a new zero from what I keep learning every day, I can't improve.
Not only do I have to be my own worst enemy, I have to be my greatest cheerleader.