I won’t phrase this properly, but: maybe you did understand recursion, but not as the mathematical or computer science abstraction you needed to learn.

I remember when it clicked for me and I wrote a function that conditionally called itself, I had this distinct sense that I already understood this concept perfectly well. I was just struggling to understand it in the way a specific programming language needed me to.

The idea of recursion is very simple, but can be complicated by the rules and syntaxes of their abstractions.

Apologies if this wasn’t your experience and I’m incorrectly assuming it must be similar for other people. I’m really not sure if it is or not.