Can't make a book if you stop on the introduction... Yeah, I know it sounds great, but how can't you keep on going, when there is nothing holding you up.
Trust me, there are too many people out there that have nobody holding them up, neither they can expect such a luxury and they still meet all demands and do stuff that you wonder how it is possible.
You won't get far if you stop on the introduction, because you don't know what is the best word for your first chapter... The best is over rated, try just writing the next instead. It doesn't need to be perfect, it doesn't need to be the best. As long you keep on going and as long you have enough time to improve, the next will always be more important than the best.
You can always improve, but it doesn't mean that you need to do hours and hours overtime, and that you have to bleed everything the last shred of your existence into the thing you are currently working on - remember, there is tomorrow and there will be people who will need you tomorrow. So you have to stop at one point and thing what is the best thing for you. Sit somewhere where you can find the inner peace, in the shadow of an old tree, that tree holds more wisdom to life than you at first will realize.
You can do so many things, by thinking them over and inventing the whole science of your thing is far more important than endlessly making one mistake after another, and using your air for nothing but enclosed circle of average. Give yourself an opportunity to be a general in your life's battles and not a husk on the sea within a battle-storm.
You are also a living being, a human who needs time to regenerate - that thing you call your body is a fabulous machine but it can also be easily broken if you are not careful. Your vessel needs time to regenerate. You would not treat badly a very expensive car, wouldn't you? Well, that body is the most expensive car you will ever have. Make sure you are the best master to it as well.