Seems simple, right? But I know of aspiring writers who are constantly working on the same book, never finishing it. If you don't finish the book, it will never be read, and you will always be stuck in "aspiring writer" mode.
Some writers have their book rejected and never try again. Rejection is part of the process. My debut book, FIRELIGHT, was rejected by all but one publisher. It ended up garnering starred reviews and winning numerous awards. So you never know!
Most writers I know possess a strange mix of insecurity and arrogance. We constantly deal with rejection, reviews (good or bad), and the pressure to sell. Yet we also believe that our stories ought to be read. The writers who hold onto that belief, despite adversity, are the ones who make it.