As I've looked back over the writing I've done for the last several years, I see no fewer than 14 first chapters, for YA to Middle Grade to chapter books.
One good thing about this is I can see that I didn't run out of ideas--I even see some concepts I completely forgot about, and I still think they're pretty good!
What I learn the most from this is that I get scared and give up quickly. I get stymied at a chapter or a particular series of needed events and I get a plot hole or I get confused and I just wash my hands of the whole mess and grasp at a new, shiny idea that looks more manageable.
I'm not "officially" doing NaNoWriMo because I don't like that kind of commitment (obviously), but I have promised myself 1,500 words a day on my current Work in Progress.
This way, there's no freaking out and flying off. If I get stuck with where I'm at, I just move on to a new scene somewhere else in the novel. I'm not writing linearly, I'm trusting myself.
I'm going to trust that this will all come together because I see 14 different abandoned works that show what happens when I don't.