Quote:
All started when I noticed some holes in the plot of my unfinished book.
In my opinion, there's your problem.
I have poured over some of my unfinished work in the past and made slight adjustments, but further reading lessened the impact of my own work and I suddenly began to doubt it's validity, and that point stuck with me...
Therefore, rather than continuously going back to already written sections, continue writing until you're finished,
then begin editing and correcting the pieces. That way, if you're ever discouraged you can look back and think "Well, I've finished this, all I'm doing is making necessary changes."