I'm exactly the same as you are, or at least I was! But I've now realized what the problem was.
I was used to sitting down and writing 1-2k words in one sitting. I was writing the full story in one sitting, even if it was 4000 words long. It was exhausting, but yet, still rewarding.
But then when I set out to write soemthing longer, like a book, the problem was I still wanted to produce that same 1000-2000 words when I sat down... And that's not an easy thing to do. It takes a long time, it takes a lot of concentration, it eats up your nerves and you're surely not ready to take on another such challenge the next day (unless you're a professional writer, of course)..
So what I've learned through the "Wild West" story that I have ongoing, that is now rather long already, and is only just well getting started, is that it's not so much about the volume you manage to put out daily, but the consistency.
I cannot consistently, on a daily basis, write 1-2k words continuing yesterday's story, because I also need time to edit, think of something new, ... ... ... you know how it works... And because it's such a hassle to produce 1-2k words a day, I often found myself with excuses or too little time in general not to write anymore, and just abandon the project (looks at several abandoned stories with great potential, all stopped at around the 4000 word mark)...
But with Wild West, I do "as little" as 200-300 words a day, it takes me ~5 minutes to write the text/general idea of what I want to convey, maybe another 5-15 to edit, for a grand total of max 20 minutes a day. There is NO EXCUSE in the world good enough for me to not take that time and write.
And I know, I know, 200-300 words doesnt seem like much... But that way I've managed to produced a couple ten thousand words in ~100 days, and they've passed reeeeal quick! Just like everything else, consistency is key.
If you cannot write 1k words a day consistently, just try less. Go for 100 words a day. Or go for "write 5-10 minutes a day"... Something that you can consistently stick to, without dreading it.
And before you know it, you'll have a long story written already :D
Looking long term, 2-3 years from now I'm looking to finish Wild West, in the same pace I'm writing it today... And sure, 2-3 years sounds like an awful long time, but I know for a fact that if I tried to do it faster, I'd give up after the first week. For sure.

Take my advice with a grain of salt, because I've never written a book myself, but still, I believe consistency>all applies pretty much everywhere in life, and I've only now truly come to understand its importance. Writing, fitness, work, ... Consistency.
A plan that you can consistently stick to is far superior to the one that is most optimal, but you cannnot stick to consistently.

And in case you and/or @spalatino write a book... I want to buy a hard printed copy, thank you very much. Signed, if possible. :P