LessWrong also has a textbook repository which contain recommendation on various subjects. Ctrl+F "decision theory" turns up 6-7 books or so.
A wild suggestion: Try voice-acting a variety of characters from TV or books.
As a non-Japanese-native, one thing which helps me shed my "foreigner's accent" is by rehearsing 'Kamehameha' and similar punch lines from Dragon Ball as a kid. Pick some of your best characters in one of their best moments and try your best to sound like them. It helps over time.
If you're willing to invest the equivalent of a few hundred Euro of your time, the Ward Audiobook Project and many others are always looking for volunteer narrators.
If you want to get a feel of the mechanics, I recommend Gilbert Strang's Linear Algebra over Khan Academy and Brilliant.
I think part of the 3b1b series is based on that book and they have similar structures. Just sit down for 1-2 hours with some graph papers + mechanical pencil and do the end-of-chapter exercises, they are designed to reinforce the concepts.
In my experience, thinking hard doesn't hurt my head directly. However, trying to think hard under stress and sleep loss does.
Whenever I noticed my head hurts trying to "think hard", it's usually a symptom that something else is going wrong.