When anyone learns something, it is not retained very well. Reason for this is material learned is not fully internalized. It has not become a second nature to you. To achieve this one must first remember the material fully. This is done by spaced repetition as was cleared in earlier post.
Once you remember it, then it should be known so well that it becomes a second nature. That is done by teaching it. The person you should teach first is you.
Imagine that you are teaching a class full of students. Let your imagination take charge. Then regurgitate whatever you have learned in that class. Since you are teacher as well as student, ask yourself questions about what you are teaching. Then answer the question in loud enough voice so that whole class can hear it. Articulate the answers good and convert it into back and forth discussion. When you finish it you will be remembering what you were teaching extremely well. Now you have already mastered what you have learned. It is now a part of your permanent knowledge bank. Essentially you have internalized the material you have learned.
The example of learning music by internalizing by Martin Taylor, it is explained in video below:
Follow the above principle to learn anything you want. As always if you need any further assistance please email me at firstname.lastname@example.org. I will be happy to respond. You may also leave a comment. I will always read and do my best to answer.