[tpool]

I’ve felt this too in my 40s. I generally don’t recommend books that might be considered self-help, but a recent book by Arthur Brooks called “From Strength to Strength” was really helpful for me to understand what’s happening to me, and how to embrace the use of crystallized intelligence in my later career. If that doesn’t interest you, you can find some interviews with him about the subject and book in a few places by search.