OK - let's get the obvious out of the way, first......the middle is the biggest section. It is also the section that most writers really struggle with. It's a bit like a marathon. You're full of excitement and enthusiasm at the start and at the end, well the thing takes on it's own momentum. You get a final burst of energy.
However, the middle is the long haul. It's where you're likely to throw the thing away (metaphorically). It is where you tangle with sub-plots and worry about holes in the plot. There will definitely be a bit where you think the whole thing is rubbish.
This is definite - you will get over this stumbling block. The horrible feeling is part of the process. When you come to reading it through, you will be surprised that this middle section is a lot better than you thought.
This is the point where a self-disciplined routine - be it 1,000 words a day, whatever, is essential. Soon, the finish line will be in sight. I promise.