Rompecabezas literally translates to "head-breaker", but it's the common Spanish word for "puzzle". It's a fun and evocative word that paints a clear picture.
In this song, the singer uses it metaphorically to describe the difficulty of moving on after a breakup. He sings, "No he podido armar este rompecabezas" (I haven't been able to put this puzzle together), implying that his life feels incomplete and fragmented without the other person, especially because the most important piece is missing.
Have you ever tried to forget someone, only to realize it's way harder than it looks? La Fiera De Ojinaga totally gets it in "Olvidarte ¿Cómo?" [Forget You, How?]. The singer is struggling to move on, admitting, "no sé si puedo olvidarte, es probable que no lo es" [I don't know if I can forget you, it's probable that I can't].
They're really going through it, especially when they hear from the other person. The lyrics, "no me digas que no eres feliz, que sólo lo ha sido olvidado" [don't tell me you're not happy, that you've only been forgotten], show how much it hurts to think the other person might be doing just fine. The singer feels like everyone else thinks it's easy to forget, because they're "desde afuera" [from the outside]. But for the singer, it's a huge challenge: "olvidar este tiene cierto grado de dificultad" [forgetting this has a certain degree of difficulty]. They even say, "no he podido armar este rompecabezas, hasta la más importante de las piezas, faltas tú" [I haven't been able to put this puzzle together, even the most important piece, you are missing]. It's clear they're still deeply in love and can't imagine forgetting them. How can you forget someone when you still see them in your heart?