It is about seeing someone who broke your heart. You've missed them to the point of losing sleep. You see them again, and they seem perfectly fine. They interact with you as if they've forgotten the history you share. I can't tell if this ex wants to hook up or just be friends, but either way, it's clear to the narrator that they are both out of touch with each other's feelings. They're fine, they don't seem to have lost sleep over you, and you realized that your missing them is a waste, because they don't miss you.
It is about seeing someone who broke your heart. You've missed them to the point of losing sleep. You see them again, and they seem perfectly fine. They interact with you as if they've forgotten the history you share. I can't tell if this ex wants to hook up or just be friends, but either way, it's clear to the narrator that they are both out of touch with each other's feelings. They're fine, they don't seem to have lost sleep over you, and you realized that your missing them is a waste, because they don't miss you.