Those of you who know me personally (have seen me at various writerly events in the past 6 months or so) have hear me expound on submitting your work whenever/wherever possible. Now I'll admit that's easier to do when you're a poet as I am. Those little nuggets take much less time to write and then to polish. But short stories, essays and then larger written work such as novellas, novels, whether in their entirety or as excerpts, should be sent out into the world as well.
By tracking submissions, rejections and acceptances, it's relatively painless to keep writing moving (out and back). There's always the opportunity to polish or edit between submissions, especially if you're fortunate enough to get some feedback. Yes, rejection is tough, but there are so many benefits to having your work interact with editors and, eventually, readers. It's the ultimate high when an editor says, "we were impressed with such-and-such."
And finally, the only way to toughen up that writer-armour is to expose yourself to the outside world. Yes, there is rejection, but it becomes much easier to swallow when acceptances happen. So, send that stuff out!