How old are you? At 39 (20 years of professional experience) I've forgotten more things in this field than I'm comfortable with today. I find it a bit sad that I've completely lost my Win32 reverse engineering skills I had in my teens, which have been replaced by nonsense like Kubernetes and aligning content with CSS Grid.

And I must admit my appetite in learning new technologies has lessened dramatically in the past decade; to be fair, it gets to a point that most new ideas are just rehashing of older ones. When you know half a dozen programming languages or web frameworks, the next one takes you a couple hours to get comfortable with.

That's one of several possibilities. I've reached a different steady state - one where the velocity of work exceeds the rate at which I can learn enough to fully understand the task at hand.

to fix that you basically need to switch specialty or focus. A difficult thing to do if you are employed of course.