The first step is to go in and read what you wrote prior to setting it down. Not necessarily the whole thing (ain't nobody got time for that) but at least the last section, or the last few chapters. This step is crucial in answering the question on
everybody's my mind, which is "what is going on here?". The only way to answer that is to read through the most recent pages from 6 months ago and sort of gently, painfully flail your way back into the story.
Okay, so when you left off, you were gearing up for a polo match in which your undefeated team was about to lose, crushingly, to their most bitter rivals. Okay, great. Let's do it!
Okay, problem. You have zero ambition to get in there and write the nitty gritty details of playing a complex game on horseback. Okay, no problem! You've done plenty of those scenes, it was getting repetitive anyway. Instead, you can build the scene around the team dynamics (which are all screwed up, which is why they're L-O-S-I-N-G), and the failed romance of two rival players. Plenty to work on there, right?
Okay, yeah, if you say so. I guess. Okay.
You are able to struggle through 2 pages worth of material and then you have obligations and you have to leave.
It takes you 2 weekends just to get through the chapters involving the losing game, something which you would have burned through in a single morning. Your Part 4 is still not quite done. You have 2 stupid chapters to get through before you can move on to Part 5, The Last Part, which is another 50 pages of material. You laugh at your previous estimation that you would get this thing done over the winter. It's already mid-December, ahahahaha crap.
You have another unrelated project you want to get to, but you refuse to leave this one hanging, so you persevere. A couple pages at a time over the course of a weekend. No wonder you can't quit your day job.