Trolling through old content from Absolute Write, I happened across the transcript of an interview from last January with Jenna Glatzer. She's actually the owner of Absolute Write, as well as an experienced writer. In the interview, she talks a lot about stage fright and how she found that affected her strongly in her prior career as a stage actor, and yet how it isn't something that has ever really been an issue for her with writing. She postulates that this is at least partly because writing is her secondary thing, while acting was her original dream.
I think that's very interesting, because I think a lot of aspiring authors hold writing as their Big Dream, and thus a whole lot of their identity is wrapped up in the stories they write. I think that might contribute to that fear of rejection that we all have--this isn't just some random job interview of which there are many more possibilities besides. This is our Dream, and there's only so many agents and editors out there. If writing is our only way to evaluate our professional self worth, then that's even worse--everyone gets rejections, and most people can't get their first book published (I read over on Pub Rants a while ago that the average might be three unpublished books before one is sold).
I don't mention this quasi-statistic to be discouraging, and neither did they. Just because that's the anecdotal average doesn't mean it will hold true for everyone, either. But no matter what, you're going to get rejections. I've never heard of a published writer who said they hadn't. So the important thing is to not let yourself get too tied up by any individual work--whether that means moving on to the next book while you query for your current one, or whether that means not being so afraid that your current book isn't good enough that you never finish writing it. This is a time-intensive business, and as anyone who has queried for a book can tell you, the actual long process of writing the book is only Part 1.
Write for yourself. Unlike singers or actors, we don't have to perform on stage, and we should use that to our advantage. Don't needlessly put yourself on stage while you're writing. I know several singers (my sister and my sister in law are both terrific), and one of their biggest problems is not letting performance anxiety affect their performances. When they perceive the stakes as being really high, there's a risk that their vocal cords will constrict, and that has a very negative impact on the quality of their singing. You never hear this in professional singers, because sometime before they went pro, they learned how to control their fear.
And I think writers face the same challenge. A certain amount of introspection is healthy and wonderful, and absolutely necessary. But before we're ever going to be a true pro, we have to learn how to differentiate between the healthy analysis and that which is just stage fright. We have to write like there's no one else in the room (and, um, there probably isn't). Writing isn't like stage performance. It's like working in the recording studio or for the camera: you have as many takes as you need (and there aren't any other staff/actors/singers waiting on you, either, so even better). If your first draft of a new scene or chapter isn't great, don't despair. You have as many revisions or rewrites as you need, and you're the only one watching or keeping count. Write for yourself.