My best advice to authors feeling Impostor Syndrome is three-fold. First, talk to other authors - you'll be shocked at how many share your anxieties. Second, compare your achievements only to past versions of yourself, not to others. Thirdly, remember that you have written - or are writing - a book. Do you realise how many people dream of doing that, and never fulfil that dream? You are an amazing, one-in-a-million person for that. And I really mean it.