Demorou comes from the verb demorar, meaning 'to take a long time' or 'to delay'. While it's a common verb, its use here is particularly poignant and central to the song's narrative.
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Ever missed someone who doesn't exist anymore? That's the feeling at the heart of Wesley Safadão's "Amor, Ex Amor." The singer isn't missing his ex-partner as they are now, but rather the person they used to be. He points out how much they've changed, saying, "My longing isn't for you / It's for someone you once were."
He suggests that while his ex might be popular, they are ultimately lonely, asking, "What's the use of having a heart for the crowd and living alone?" The singer waited, but his ex took too long to realize what they had. Now, it's too late. Someone else has stepped in to care for him in a way his ex never did. He calls them his "love, ex-love," declaring that their past relationship couldn't have been true love if it ended, and that his ex's actions were like asking "to lose me forever."