Whether you're at 'Once upon a time...' or 'The End' of your book project, at some point doubt is going to settle upon you with all the weight of a mountain. What to do? Well, revisiting the architecture or structure of your story can help. Have you included all the required/expected beats? Are they in the right place? Jami Gold's website (if you haven't already found it) is an amazing resource, covering as it does the craft of writing and story in a lucid, engaging and accessible way. Her beat sheets (freely available) have got me out of many a hole. So, so useful. So, so comforting.