Writer Rachel Cohn (Nick & Nora, Gingerbread, and so many more fun reads!) says:
My favorite adage I often offer fellow writers is the very thing I most have to remind myself: Tell your inner critic to STFU. (Put more nicely, tell that devil on your shoulder to bugger off.) Get out of your own way and trust your characters. While you're writing a draft, don't agonize over what others may think of it. More importantly, don't second guess your ability to bring your characters to life. Of course you can do it -- that's why you had the idea in the first place! Take a breath -- and give that breathing space to your story.