Die unendliche Geschicte was written by Michael Ende and published in 1979. Movie rights were so quickly snatched up that the cinematic version was released in the summer of 1984, when my dad took me and my brother to see it as part of a double feature (I was three). Although I finally read the book years and years later, I find myself most devoted to the movie, and so it is the movie that I will be discussing here. Note: I'm assuming you're familiar with the story.
I'm pretty literal-minded, so symbolism can be difficult for me. You may have to forgive my being unable to see the broader picture or theme if I have missed the mark. For example, one layperson's review of TNES said that it is clearly about the fantasy world we have to give up in order to move into the world of adulthood. I have always interpreted it to be about humanity's loss of meaning and hope as a result of the broken promises of modernity - but that may be because that is how Ende framed it. Although children are the protagonists throughout, and it is allegedly a children's book, he does not say the Nothing is encroaching due to the end of innocence and childhood. He says it's because "people have begun to lose their hopes." So I have a hard time detaching from that.
Several years ago, after probably my twentieth viewing of the movie, I finally noticed the duality that springs up at several points, even though, in my final opinion, the movie's narrative does not hinge on it. This is likely due to the concessions made in the script in order to make the first third of the book filmable. I think that the instances are worth acknowledging.
Morla the Ancient One - Morla (aka my spirit animal) is a giant turtle on whose back exists the Shell Mountain. Atreyu treks through the Swamps of Sadness (losing his horse in the process) to ask her if she knows of a cure for the Empress's illness. When she refers to herself as 'we' and Atreyu asks if there's anyone else there, she says "We haven't spoken to anyone for thousands of years, so we started talking to ourselves." She, perhaps due to her persistent loneliness, has become pretty nihilistic, repeating the line "Not that it matters, but yes" and also this timeless gem: "We don't even care whether or not we care." I realized it's probably no coincidence that her home base is a swamp where people and animals drown when they give in to its palpable sadness.