Acortándome is a form of the verb acortar (to shorten), meaning 'shortening me' or 'shortening for me'. It's a very dramatic and poetic word choice.
In the song, Chayanne sings the devastating line "Y tú te vas, así como si nada, acortándome la vida" (And you leave, just like that, shortening my life). He uses this powerful hyperbole to express that the pain of the breakup is so profound it feels like it's literally killing him, a truly unforgettable way to describe heartbreak.
This song is a massive power ballad about being completely blindsided when someone leaves you. Chayanne sings from the perspective of a man whose partner is walking out the door, and he just cannot understand it. He starts by saying, "Nunca imaginé / La vida sin ti" (I never imagined life without you), making it clear he saw their future together. He feels he was the perfect partner: always honest, always available, and gave everything he had. He's totally confused, asking, "Si es que te he fallado dime cómo y cuándo ha sido" (If I have failed you, tell me how and when it was).
What really gets him is how casually she seems to be ending things. The chorus hits hard with "Y ahora tú te vas / Así como si nada" (And now you're leaving / Just like that). He feels like his world is ending, that she is "acortándome la vida" (shortening my life), while she won't even look him in the eye. He's left wondering what happened to all the promises and the love she swore to him. It’s a dramatic, heartfelt plea from someone who is lost, confused, and desperate to understand why a love he thought was forever is suddenly over.