Derrumbo comes from the verb derrumbarse, which means 'to collapse' or 'to crumble down'. It's a strong, dramatic verb that perfectly captures the intense emotion of the song.
In this context, David Bisbal sings "me derrumbo" (I crumble down) to express how he feels completely broken and lost without his loved one. It's a powerful metaphor for emotional devastation, making it a very memorable and impactful word.
David Bisbal built "Me Derrumbo" around a promise the singer can't keep. He tells himself, "Yo quise imaginar que sabría esperar" [I wanted to imagine that I would know how to wait], but the waiting breaks him anyway. Every chorus lands on the same admission: "Porque sin tu amor me derrumbo" [Because without your love I collapse] and "Soy solo carne y huesos yo sin ti" [I'm just flesh and bones without you] — he isn't dressing up the heartbreak, he's saying plainly that he falls apart.
The bridge tries a different argument, that "La distancia entre los dos hace fuerte nuestro amor" [The distance between the two of us makes our love strong], but he can't hold onto that idea for long. By the final chorus he's back to "Es un infierno en mi interior" [It's hell inside me]. It's a song about the gap between what you tell yourself to get through a day apart and what you actually feel once the day is over.