Have you ever worked on something for years? I mean really poured your heart and soul into it, giving it all you've got. I can tell you that, personally, I have been working on a book for the past several years and I am nearly finished. But now I am absolutely terrified of actually showing it to anyone because after putting so much work into it, I don't think I could handle any kind of rejection. It would be devastating!
Well, think about J.K. Rowling, who worked on the first book for over five years and then got rejected by twelve(!) different publishers. Can you imagine getting so many rejections and still forging on? I'm not sure I would have made it past two or three rejections (maybe not even one, actually). She was writing the best-selling book series in history, and she still got rejected over and over. It goes to show you need to believe in yourself because most "professionals" don't know anything!