Me arrepiento translates to "I regret it". It comes from the essential reflexive verb arrepentirse, which means to repent or to regret.
This phrase is the emotional core of the song, appearing in the iconic hook: "Si una vez dije que te amaba, hoy me arrepiento" (If I once said I loved you, today I regret it). Learning arrepentirse is key for expressing a change of heart or remorse, making it a very useful and emotionally charged verb to know.