I wouldn't say years, but it took quite a few weeks, maybe even months:

The idea that the expansion universe is akin to the surface of a balloon expanding (albeit in 4D).

I don't know why. It is so absurdly simple in my mind now. But when I was first told this (after naively asking where would the center of the expanding universe be), I just couldn't put my head around it, it seemed absolutely gibberish to me at that moment.