The great end of life is not knowledge but action.

Understanding is a means and gets promoted to an end surprisingly often. You can read, analyse and thoroughly comprehend a subject while nothing whatever changes in your conduct, and from the inside that feels like progress because the mind is genuinely busy. The test of what you know is what it makes you do.

Pick a subject you have studied for a while and name one thing you now do differently because of it. If nothing comes to mind, the learning is still sitting in storage. That is not wasted and it is not finished either, and finishing it takes an action rather than another book.