Beginnings are easy, because I don't start writing a book until I know how it starts. Endings are easy because I know when I've gotten to the end. It's instinct. But The Middle? The Middle is a flashlight beam in the woods that only reveals a portion of the path at the time. Made worse by suspicious sounds coming from an ambiguously located but obviously nearby location.
I know when I'm in The Middle, because I started grumbling about how awful this was. I looked at my wordcount and judged it against what books by me usually total and went "oh, it's not the story. It's just The Middle." Then I remembered that there's a light novel sitting on my shelf and several finished manuscripts that each did their tour of duty in The Middle, and each came out the other side and found the end. This one will, too, and how ever awful it may be now... I'll just need to finish it. I can do something with a finished thing.
Surviving The Middle is learning to love the story, despite that it's not what I thought it was. It's when the story becomes the beginning of what—after significant revision—it's going to be.