I finally figured it out. Yes, muscle fatigue plays into my not wanting to keep knitting this for too many hours a day–and yet, after working on it I do want to keep knitting, just something simple.
It isn’t the complexity. I can handle separating and untwisting tangled strands just fine.
It’s that about half the stitches require active decisions on the spot as they make their way across the needles.
I’m finding that the further along it gets, the easier that part is, too, because I can see how it’s coming out and I really do like what I’m doing. 