When it comes to practicing piano...and creative work in general...there's one thing that holds us back.
We don't define what "done" looks like.
So, we may spend years writing a book that never gets published. It only "progresses," as we say.
Or, we get frustrated in our practicing. Because we spend some time at the piano, hope we improved some, and continue on.
But with any project, we must define what "done" looks like.
Your book isn't finished until you've published the book. And all your work should be heading in that direction.
Your practicing is successful when you reach the goal you set before you started the practice session. A goal that an 8-year-old could understand.
Take someone who has no idea what you do: If they can tell that you've finished something, then you know you've defined "done" correctly.
And after "done" comes "next."