Whenever anyone asks me what’s the single most important thing they can do to sell their book, I always tell them to write the next book.
This is one of the fundamental rules to follow in the quest for publication. Every time you add a story or novel to your inventory you increase your odds of selling and you get vital practice that will make you a better writer. The vast majority of writers don’t make the break on their first story or novel. The first novel I sold was my 4th, but I didn’t sell it until I’d already completed 7 and having my editor read and really like #6 had a lot to do with selling #4. I’m at 9 now with only 1 in print and 1 forthcoming, though it’s likely I’ll sell at least 6 of the remaining 7. My short story career is similar, though I’ve been publishing longer in shorts and have now sold something in the neighborhood of 30 of 50.
It’s easier to sell if you have more stuff out, because you have more stuff, because it familiarizes editors with your name, because it demonstrates that you’re not a one story writer, and because with every story you get better.
So, practice. Write the next story. And remember one of the best things about being a writer as opposed to many other kinds of artist is that you occasionally get to sell even your practice work.