The book you care about may not be your best
Your first book is the hardest to write, for several reasons. The first is that, you're learning how to do everything all at once... so the book takes a long time, and you get better as you're writing it. So it's messy, and very hard to clean up and polish off successfully. But it's also hard because you care about it too much.
It's probably the one big life goal and project you've been thinking about and working on for several years. You're more attached to the outcome, which will make it harder for you to let go and kill your darlings. If you're feeling stuck, take a break and write something fun, as a personal challenge, but focus on finishing and publishing it. You can use a pen name if you have to. Just learn to write quickly, finish and hit publish. This is easier to do if you don't feel a personal investment. It will also make you much able to absorb feedback and criticism without taking it personally.
UPDATE: I'm about to start talking a lot more about the craft of writing books, and posting articles everywhere to build up my author platform. You may not see those unless you follow me or visit my blog: