I'm smack dab in the middle of a four book series, but this weekend, I'm going to start book #3 and put myself over the hump.
Most writers can relate to what I'm feeling right now. The book you will begin writing will never be as perfect as it is now. No matter your own pre-writing process - "total, detailed, multi-paged outline" to "fly by the seat of your pants" and everything in between - you've solved all the foreseeable problems, and while you might not know exactly how you'll be getting from point A to point Z, you'll get there and OH MY GOD IT'S GOING TO BE GREAT!
Even if you've completed a manuscript before, and therefore you know that all of the above is TOTAL BULLSHIT, it's still a fun time. Because maybe, just maybe, this time it's going to be different. This time, everything will go smoothly. No stupid, fucking character is going to get out of line and mess shit up. Or, perhaps even worse, no character will become so lame and boring (yet somehow you've hinged your story on him/her) that the whole manuscript will fall apart because you keep falling asleep while writing it or even thinking about it. No, no. Not this time. This time it's definitely going to be different.
It won't be.
The journey an author goes through when writing a novel is rather predictable. Case in point, a little over a year ago, Kristan Hoffman sent me this "novel chart." (You can click on it to enlarge if you so desire.)
I just wrote book #2 this past winter, so I can still clearly remember my "dark night of the soul" with that one. You never do come all the way back from it, as the chart clearly shows. Well, you don't come all the way back from it until you start the next book. And let me tell you, book #3? It's the greatest idea I've EVER had.