I think around the 75-80% mark it feels a slog. You're not within touching distance of finishing, and you've been doing it for so long. I usually also find there's a bit round there that takes far longer to write than I pictured.
But think of it this way... You've only 20% left- that's a quarter of what you've already done.
Yes, keep going. I am editing/re-writing some of my book and I am about half way (around 40,000 words) and haven't flagged so far but fear this may well set in. I am approaching third trimester of pregnancy and this has probably a lot to do with it but I am determined to plough on as the thought of writing with a new born/ toddler on the loose seems very daunting - trying not to think that far ahead and dwell. So yes, keep going x
I'm new here mousemoose but just want to cheer you on. You've achieved so much already, and I echo those who have already said there's only a small percent now to be done. Deep breath. Cup of glass of favourite tipple. Write away without thought of organisation or censure at this point - you can do both later. Picture the cover of your book and your name embossed there and on the spine