... the feeling of flow is different than the feeling of getting better. If all you seek is flow, then you’re not going to get better. There is no avoiding the deliberate strain of real improvement.
Writing gets harder as I get better. But that's okay, because it means I'm getting better. I'm writing a techno-thriller right now, and it's like pulling teeth to write, but I am determined, because see above. Also, it's all kinds of fun, because nobody said it couldn't be hard and fantastically enjoyable at the same time.
I highly recommend the blog in question (Study Hacks), by the way, as I find it to be insightful wrt the learning and practicing process.