This quote was at the end of the article I've linked to in this post. It sums up the world of writing screenplays very well, but it also works for writers as well:
Marc Norman likens screenwriting to a phrase he used to hear that "The movie business is a place where you can make a fortune, but you can't make a living." Meaning every once in a while, someone comes up with a pot of gold. But, try to do it again or try to do it every year. Writing is difficult because it offers you no solace for failure. If you get frustrated and leave, no one will cry.
The article can be found here: How writer Marc Norman went from the Mailroom to winning an Oscar bit.ly/U86uIG
It's worth reading.