A big part of that is context, I think. When we were children, we didn't have the knowledge or developed brains to recognize these things. And the lossy nature of our memory leads us to skew towards remembering things in more idealized manners, probably because it is easier to recall things as "concentrates" of reality. The parts that we, as adults, recognize as problematic don't tend to be remembered as significant because, when making the initial memories as children, we lacked the context to flag them as such.

I think being an adult is realizing that I don't love Harry Potter as much as I used to, because (I can't believe I'm saying this) I've finally outgrown it. It's time to move on.

On other hand, this realization frees you in a way and may potentially inspire you seek out or create another piece of art to love (and potentially share with others). While I disagree with a significant section of the population and believe that art is inseparably and indelibly linked to the artist, it is important still to be kind to our past selves and not judge them for what they didn't know. That still doesn't entirely soften the blow of "breaking up" with a piece of art that one has loved but, it can help with accepting it.

Being a grown-up is painful.

It's also joyful, terrible, wonderful, enraging, sorrowful, and countless other feelings and possibilities. I think it's beautiful, even if not always comfortable. And the uncomfortable bits provide contrast to the positive, letting them seem to shine a bit more brilliantly. Though, that could also be the near-pathological optimism that I adopted to cope with depression in my younger years.