There are days when nothing sticks in your head and you can’t concentrate either. Never mind! There can be unproductive days! If you really can’t do anything, try to make use of the time you have set aside for learning.
Bad-learning days, when you have trouble concentrating and learning, can still be used productively. Exactly on such days it is advantageous not to make the learning time dependent on achieved goals. Nevertheless, at the end of the day you can see that you have learned.
For example, read articles that interest you or chapter introductions in books on the subject. Copy summaries from books or from yourself. Or google a topic or keyword on the Internet and read a few entries about it. This will help you keep a topic in your working memory, even if you don’t learn anything specific! And you will see, the next day learning will be easier for you.
On such learning days it is advisable to repeat topics that you have already learned and can do reasonably well. Seeing what you can already do has a motivating effect. So try to use your learning time wisely, even if you don’t learn anything new.
Maybe you need to increase the number of rewards for learning goals on such days. In addition, you should take the following day (half or full) as a time-out if possible. After that, learning will be easier again.
It is very bad for your motivation if you stress yourself out! Everyone has such days and that’s okay.