Me arrepiento means 'I regret it'. It comes from the reflexive verb arrepentirse, a powerful word used to express deep remorse or regret for something you have done.
This phrase is the emotional core of the song. The singer declares that if he once said he loved someone, 'hoy me arrepiento' (today I regret it), marking a dramatic turning point from past love to present-day regret.
We have all said things we wish we could take back, and Carin Leon is not holding anything back here. In this Regional Mexican track, he tells an ex that loving them was a total mistake. He lays out the situation directly: "Yo te di todo mi amor y más / Y tú no reconoces ni lo que es amar" ("I gave you all my love and more / And you don't even recognize what love is"). He admits he humbled himself completely with "Yo me puse dispuesto a tus pies" ("I put myself willingly at your feet"), only to get treated with contempt in return.
His reaction is blunt and funny in how honest it gets. He belts out, "Si una vez dije que te amaba, hoy me arrepiento" ("If I once said I loved you, today I regret it") and chalks up his past feelings to temporary madness: "No sé lo que pensé, estaba loco" ("I don't know what I was thinking, I was crazy"). He drops a great Spanish idiom with "Ese error es cosa de ayer" ("That mistake is a thing of yesterday"), which means that mistake is firmly in the past. He knows they will try to come back eventually, but as far as he is concerned, he is done.