Arrepentí is the past tense of the powerful verb arrepentirse, which means "to regret" or "to repent". It's a word packed with emotion, used when someone feels deep remorse for a mistake they've made.
In the song, the singer looks back on the relationship and confesses his sorrow, singing, "Si fallé me arrepentí" (If I failed, I regretted it). This one word perfectly captures the song's heartbreaking theme of longing and wishing you could undo the past.
This song is one big, dramatic plea to an ex. The singer is absolutely convinced that his former love has made a huge mistake by being with someone new. He opens with the super intense line, "Duele cien veces más / Que si se hundiera un clavo en mi piel" (It hurts a hundred times more / than if a nail were stuck in my skin), just to make sure we know how much pain he's in every time he sees a photo of her with her new partner. He's done pretending he's okay with it and is laying all his cards on the table.
He spends the whole song questioning her decision, certain that she can't possibly be happier with this other person. He asks, "¿En qué punto fue que te perdí?" (At what point did I lose you?), because he feels like he gave her everything. He's so sure of his love that he repeats the ultimate claim: "Nadie te amará como yo a ti te amo" (Nobody will love you like I love you). It's a classic pop song about heartbreak, full of confusion and a refusal to let go.