Here's a bit of a lesson that I posted elsewhere so I'll plagiarize myself and repeat it here. It describes the failure of my novel attempt a few years ago when the thing fell apart after about 100K words.
The novel I nearly finished while in New Orleans was doomed from the start by serious structural flaws in the plot. A plot must be thought out and must survive a series of questions that mostly start with 'why.' Why must that be that way? Why do the characters do that? Why would the villain do that and not just shoot them? Why is anybody bothering to do anything at all?
I had an implausible premise from the beginning and didn't realize it until too late. The further I got along the more difficult it became. I couldn't think of any solid reason that could recover it. Also, I had the hero separate from the rest of the pack and head back to get revenge on the villain. Soon after that I lost the structure I needed and couldn't recover.
Stories are presented in three acts with the hero going through four phases: orphan, wanderer, warrior, and martyr. He or she goes from reactive to proactive at the midpoint of the story. The timing and spacing of these phases need to be hit at the right times because that's what readers expect after years of training with other books as well as movies. You miss these marks at your peril and I missed them terribly. Combine that with the implausible plot and the thing fell apart, becoming nothing more than a series of scenes with no arc and no compelling reason to root for the hero.
That, by the way, is a textbook way to kill your novel.
And learn something all at the same time.