On Learning:

I skimmed Barbara Oakley's book on Learning.

There are two points worth mentioning:

If you are ambitious to learn something new, in particular a subject that loads heavy on maths, it makes sense to create a learning plan; i.e. you should ask yourself, how many days or weeks you're able to invest in learning, and what time span per day is available. You should check after each day, if you invested the scheduled time frame. It is absolutely important to create a realistic plan. So, if you are not able to invest as many hours as planned, use that feedback to fix your plan, i.e. to create a more realistic plan. It makes sense to subdivide your learning effort in 25 min or 30 min intervals (use a timer), with several minutes for relaxation between these intervals. This should improve concentration.

Secondly, you only know something if it's really in your head. If you restrict your efforts to reading materials, perhaps an illusory feeling arises, that you have learned something new. But for learning it's absolutely essential that you are able to recall / to consciously reproduce and reconstruct the learned material. Unless you can explicitly talk about learned material, you haven't integrated the new information into your store of declarative knowledge.