(disclaimer: I write scripts, so if you write fiction, relevancy may not be there for you. But I hope you get a nice laugh anyway.)
I've learned that it's harder than it looks.
I learned that a perfect masterpiece can always be improved. And may contain typos.
I learned finishing that first draft ain't finishing nothin'. (Pardon my colloquialism)
I learned 200 to 1 are not winning odds.
I learned that when the readers say they want solid structure, likable characters and an exciting intro, they aren't kidding. If they are a producer, it's "they aren't #%@&!*g kidding, you %@&!in %@&!."
Speaking of reading, I learned reviewing is harder than writing. Also, the other writer's deluded incompetence is not your entertaining quirk.
I've learned that while it's fun to fly around the country pitching your work, it's not worth flying coach. Unless you have a shot at actually making the sale.
I learned that when you really work hard, and market yourself, and polish your work to the absolute best it can be, the certainty of your crushing failure may actually become less absolute.
I learned you can come back to edit these. So I'll think of something else later.