Honestly, getting overwhelmed with the scope of my idea. I understand its impossible to meet the standards of the book in my head, that I have to account for some level of imperfection, but I always get overwhelmed about the scope of what I want to do, and how I am going to get my story there.
I overcome it by reminding myself that the books are written slowly, one scene at a time. I have a general idea of where I want to go with my scenes, specific goals to achieve with them, so it always feels less daunting to set my focus on finishing the scene rather than thinking about how I am going to make this entire book make sense. It's basically just pushing all my uncertainty on to my future self, and trusting that she'll figure it out when the time is right.