I've hit the "it's getting hard" point in my WIP. I told myself I have to keep chugging through, no matter how painful, at least until I get to 40k. Then I can stop to reevaluate. To be honest though, I just want to finish this no matter how awful it is. I can't keep writing just the beginnings of books for the rest of my life. I need to learn to STFU and just write the damn thing.
It helps that there are two viewpoints in this book, because when I get bored of one I can switch to the other. And I want to follow my whims more. I tend to write chronologically, because where else is a better place to start than the beginning, but if I have a scene in my head, there's no reason to keep forcing out that other scene I was working on earlier. Write what I can see, what I'm most excited for. And don't worry about transitions or any of the crap. Start with the framework, get out those scenes most important, trim away the irrelevant crap, and fill in the spaces later. It's a constant cycle of trimming and adding.
I really liked this tweet from Lev Grossman: I'm beginning to think that most of writing a novel is just not losing your nerve. That is the only thing I need to be thinking about while working on this zero draft. Well, also this (wallpaper here). And this.
My definition of zero draft: all of my word vomit that occurs before I have a full arc. This includes the scenes I know I'm going to trash. First draft: all discarded scenes have been discarded, remaining scenes in shifted order, add a few transitions or new scenes. Then the first round of real edits can start, moving the manuscript into draft two.