If you're looking to write a best-selling self help book in the near future, you could tell people to spend more time living in the present. That's always been a message people are eager to hear.
We like to read about it because we don't do it. Instead we spend a lot of time depressed about the past and worried about the future. Or just distracted.
Effective practice obviously requires being focused on what you are doing in the here and now. If you are worried about what you are going to make for dinner you aren't concentrating. If you are upset about the notes you missed the last time around and all you can think is "don't miss them" you will miss them.
Really letting go of useless baggage is a cultivatable skill. It can take some time. But the real talent is not in the over-simplicity of popular advice, it is in being able to tell what will be useful.
For example, the other night when I couldn't sleep I used the time to rehearse a lecture on Beethoven I'll be giving next week. My brain wanted to keep running and I let it. I had probably had too much caffeine that day. And, while worrying about something I have to do next week is counterproductive, preparing for it is not. Also, a lecture is something you can actually rehearse in your head while you lay in bed, unlike practicing an instrument.
There are ways to accomplish that, however. Once you've got a piece to a certain stage in preparation, you can go over it in your head. But simply worrying about it might not always be a bad thing.
Last week I found out rather suddenly that I would be participating in a master class this month, playing a tricky 20th century French piece on the organ. In the midst of various other preparations, that is just another thing to add. And I wanted to get a quick start on it. But between Monday night and Tuesday morning all I could do is worry about it.
I think that may have helped. I had played the piece before--spent two weeks on it exactly a year earlier. And although I couldn't consciously remember any of it at the time, I think sending signals to my brain that I was going to need it again may have actually helped my research department locate the files.
Anyhow, the handful of days I had to work on it before the next interruption was enough to get started. It's partially memorized, and feels familiar under the fingers.
The brain is a fascinating place. It doesn't come with a manual, and learning how to use it to maximum advantage takes years. Unlike the popular bromide, we can't just live in the present. The past and the future are always part of us. We can always learn from the past and we had better prepare for the future or we won't like it very much when we get there. But there is obviously a place where we have stopped enjoying the once place we actually are, and our obsessions are unhealthy. Figuring that out is what the other 299 pages of your book are for.