In the course Learn How to Learn, there are powerful mental tools to help you teach heavy objects, where I learned of the Pomodoro technique.
This technique recommends 25 minutes of concentrated effort followed by a short duration of mental relaxation (25 + 5 + 25 + 5 + 25 + 5 + 25 + 15).
https://en.wikipedia.org/wiki/Pomodoro_Technique
Worth a try!
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