That’s an old trope, made prominent by some New Age guru types. “It’s when you feel you aren’t making any progress that you’re growing the most!” It’s a good thing to tell yourself, especially when you’re feeling down about how slowly your work is going, or how terrible it all seems, right now. Conversely, it’s good to stay a bit humble about it when you think it’s brilliant (and I hope you do, sometimes!). An even temperament is the machine that drives a steady flow.
And there’s some truth to the trope, in my experience, but I’d say it’s more true that you don’t know how well your work is progressing in the time you make it. Look back on last year’s work and you can see good stuff and not-so-good.
But we are poor judges of today’s work, yesterday’s work, even last week’s work. It’s not important how you feel about what you just made. Remind yourself that future you gets to evaluate. Present you has one job: keep making it.