Please ignore the embarrassing click-bait title on this video:
I've known this for a long time now, but I've rarely heard someone else express this so clearly in such a matter-of-fact way.
"You can control what you think. You can control how you think. Therefore you can control who you are."
When I'm learning the piano and tackling new challenges, this is effectively what I'm doing but on a much smaller and less ambitious scale, but hey, it may be worth thinking bigger.