Your questions are going to be hard to answer, or, rather, your questions not so much...The advice will be hard to give.
How do I let it go and keep working, realizing that things will have to change to make the front and the back match up? I write the next sentence, the next paragraph, the next scene. I had a point early on in BROKEN MIRROR where I realized that I had all of the important details laid out in my mind in one nice straight line. I knew what would happen and on exactly which day it would happen (the narrative unfolds over the course of two weeks). I just didn't know how to connect the dots. How to compel the "action" (such as it is) from one place to another. It was taunting me, kicking me in the teeth, keeping me up at night. Finally, I told myself that it didn't matter HOW the next scene was arrived at, just to get there and write it. I'm going to be taking it out of the notebook and typing it later, right? So just put in some bloody filler already and get on with it.
There are a couple of places where the original sorta looks like this: "John Doe experienced a pivotal moment in his life. Later he ordered up a pizza and watched a movie on TV. He went to work the next day and was bored out of his skull. While he was at the grocery store he saw someone he knew, they agreed to meet later. John did a few more things, went to bed. After work John went out for drinks with the person from the grocery store. Another important thing happened to John Doe that further alters his view of How Things Are." Then, just to be sure I don't slavishly type all that in one night while I'm getting into the bourbon a little extra (hooray for Christmas!) and/or exhausted from work, I make a note on my envelope in the back. "Clean up what John was doing between Point C and Point D."
Now, all that said, I've really been wrestling this past week with bringing the first and second half of the book together. The voice is the same in both halves, but the tone is very different. The longer I wrote the book, the more I was able to get inside what was going on. I became much more apt at putting things down in the bleak, stark manner necessary for the subject I'm dealing with. Now I need to combine the two halves, which is difficult because there are two options open for me: either re-cast the whole first half in the same tone or, more likely, carefully re-cast certain parts of it so that there exists a bridge between the two tones, allowing the reader to begin with one state of mind and end with another.
So my advice (hey, look, we might be getting on to something useful finally!) will be as follows. But I warn you that it's the same advice I give to fencing students, so beware. Think about fencing writing whether you're on the strip at your desk or off it away from it. Don't only be a writer when you're writing, be one at all times. Mowing the lawn is a dull chore, it's mindless work. I love to do it because (A) no one can talk to me while I'm doing it and (B) I can think about whatever I want as long as I keep the mower in a straight line. The description of a character who turned out to be much more meaningful than I could have planned came to me while mowing the lawn. It was just a description, just a way to provide some filler, but it worked. It fit.
Were I in your shoes, I would take the first six chapters and lay them off to one side, open a new notebook/file and write Chapter 7. When I got to the end of Chapter 7, I would write Chapter 8. When I got about, oh, halfway through with Chapter 8 I would print off the first 7 chapters and read them all in one go. Does Chapter 8 still make sense? If yes, great. If no, think what which part needs work, the first 7 or number 8.
I'd sit down and listen to The Other Ones' The Strange Remain (Yes, I have that album. It's over there next to Sinatra and Armstrong. Underneath Pearl Jam.) and think about what I've written so far and what's coming next. Don't actually write your story right this second; rather, reflect and, if you need to write something, write what you think is wrong and what you expect the final version do. Not how to correct it, because it might not be wrong yet, but what you want the outcome to be.
I think the key phrase to take away from that last sentence is "it might not be wrong yet". Creative works are notoriously fickle. Just because it doesn't seem right to you right now doesn't mean it will be wrong later. It could be your subconscious steering you oh-so-slightly towards a path that will turn out very profitable.
Okay, I've really gone on here. Forgive me. If it makes sense to you, bless you. If it helps you, I'm grateful to have been of service. If it's the ravings of a madman, well, that's always possible, too.
Il en est des livres comme du feu de nos foyers; on va prendre ce feu chez son voisin, on l’allume chez soi, on le communique à d’autres, et il appartient à tous. --Voltaire