Man, unlike anything organic or inorganic in the universe, grows beyond his work, walks up the stairs of his concepts, emerges ahead of his accomplishments.
A lot of people change for good. Some people just fall off. Just trying to progress in anything, no matter what you're doing, I feel like any progression you make... some people aren't gonna be around you that were around you.
Abandon the idea that you are ever going to finish. Lose track of the 400 pages and write just one page a day, it helps. Then when it gets finished, you are always surprised.