When you need to move a story forward, it's tempting to start looking backward. But this is a slippery slope, and it might keep you from finishing that book.
Don't Look Down
You know that moment in the movies when Character B is walking across some narrow piece of something-or-other over a dangerous chasm of doom -- and they're just one freefall away from being at the bottom of it? Character C, like an idiot, always hollers out "don't look down!" Inevitably, Character B looks down. More than half the time, it makes them fall or otherwise screw up what they're attempting to do.
So the last thing I want to do is tell you not to look back when you're writing a book...but don't. You have to keep writing forward. Remember that it's just a first draft, and doesn't have to be perfect. It's not going to be perfect in the first draft anyway, no matter how long you take to write it. In the first draft, you may not have all the logistics worked out. Certain plot points may not be cohesive yet. Certain scenes might be too short, or too long. But that's okay, because when it comes to the first draft your only job is to write.
You can always work out the details later. Always be moving forward in the first draft. If you get stuck, you may want to go back and read that last few chapters just to get yourself fired up again. But don't start correcting errors, or you'll ever stop. It's okay to read the first draft before it's finished -- but don't start editing. Write, write, write until the first draft is done. You've got all the time you need to perfect that writing, but you've got to get the writing done first.