Desmadre is a versatile and informal Spanish word that literally translates to "un-mothering" but is commonly used to mean a "mess," "chaos," or a "disaster."
In the song, the artist sings, "Ay, espero que se acabe este desmadre que llevo dentro, dentro de mí." Here, desmadre refers to the internal turmoil, emotional chaos, or difficult situation he's experiencing. It's a strong, colloquial term that conveys a sense of being overwhelmed and wanting an end to the internal struggle.
Calle 24's "No Soy Pa Ti" is a really honest and a little heartbreaking look at a relationship that's just not working, even when there's still love. The singer knows he's hurting his partner, who's been "cansada esperándome" [tired waiting for me] and "llorándome" [crying for me] for months. He's carrying a lot of internal conflict, a "desmadre que llevo dentro" [mess that I carry inside], and it's making him question everything.
He admits he's doing damage, "Te hago daño, lo sé" [I hurt you, I know], and even hurting himself by thinking about her. It's a tough spot because he's not sure if he'll ever really commit, saying "yo no sé si voy a volver" [I don't know if I'm going to come back]. He even tells her, "Ya no estés pa' mí" [Don't be for me anymore], encouraging her to be happy without him. It's a sad but very real song about knowing when to let go for everyone's sake, even if it's hard.