Ask any military vet what an AAR is. They will tell you it is an essential learning experience. Same goes for poker.
A notebook to keep track of your thoughts, results, and key points is essential for improvement.
Every 90 days or so, have a trusted friend review your notes to see if they spot trends, both good and bad. Then take those items to your coach or teacher for them to suggest ways to improve on the bad as well as keep the good stuff going. Even when you do a review with your coach, let them see your notes as well. An extra pair of eyes for review is always a good thing.
Another thing I have always kept in my notebook is hand histories. I use the same review process as above. Something else I have done is to put my histories in a 3 ring binder. Same goes my notebooks except that I only keep 5 years of books.
Poker is evolving so fast that notes from 6 years ago may not be valid today. A great case in point are my original notes from when I learned The One Hand at a Time System in 2012. Several of the concepts, that if I used them today, the results would not be pretty. So, I adjusted them to today's game.
This goes directly to one of my base tenons. Read. React. Adjust. Adapt.
Notebooks allow you to do just that.
READ the situation
REACT to the situation. Take appropriate action.
ADJUST to the new dynamic. Periodic reviews are key.
ADAPT your game to the improvements your periodic reviews suggest.