How careless of me!
Today, I found out that iTunes was copying all my music into its own folder, and automatically organized the music by artist. This meant that I had a duplicate of every song! No wonder I was low on memory… I had ~56 GB of duplicated music. I promptly deleted it. All is well now, and I now have more memory! It’ll be occupied by One Piece anime, which, by the way, I’m on episode 455.