I've been trying to think of some words of wisdom to offer writers who, like me, often get stuck (technical term - writer's block...).
It happens to everyone and can make you feel pretty isolated and inadequate when everyone else around you is forging ahead.
What I would advise is:
Find a good group of people who you can go to for honest support and advice when you need it (this works both ways and you also have to be there for them when they need you!)
Even if you are blocked, try and write every day. It doesn't have to be about the book you're working on - what about some poetry? Your life and how you're feeling? You'll be surprised what's lurking in the back of your mind and something could be the trigger that gets you moving again!
Read as much as possible. This not only gives you an idea of what is going on in the market, it will increase your word power and give you fresh ideas for your work.
Try to relax - ideas will come to you and you will finish that book!