"Tanto" opens and closes on the same line: "Tanto he dejado aquí" (I've left so much here). The singer has poured so much into one place, or one person, that "ya no sé cómo volver" (I don't know how to go back anymore). It is a breakup song told from the middle of the wreckage.
The chorus is a string of tired questions: "¿De qué sirve conseguir lo que falta?" (What's the point of getting what's missing?) and "¿De qué sirve corregir la distancia?" (What's the point of fixing the distance?). By the third round it shrinks to "¿De qué sirve resistir?" (What's the point of resisting?). That shrinking is the whole mood of the song.
Watch the neat swap between the verses. First it is "Fui uno en un millón" (I was one in a million), followed by a dig: you may never know anything worse. Later it flips to "Fuiste uno en un millón" (You were one in a million) and "nunca voy a conocer algo mejor" (I'll never know anything better). Same sentence shape, opposite feeling.
The bridge looks back: "Siempre supimos nadar contra la corriente" (We always knew how to swim against the current), but now "es mejor olvidar que tentar la suerte" (it's better to forget than to tempt fate). Tentar la suerte is a handy idiom for pushing your luck.