Me arrepiento comes from the reflexive verb arrepentirse, which means "to regret" or "to be sorry for". It's a powerful, emotional word that expresses deep remorse over a past action.
In the song, Aitana sings, "Dile que en la noche a veces me arrepiento" (Tell him that at night, I sometimes regret it). She uses this phrase to confess her lingering feelings and reveal the profound regret she feels about ending their relationship, making it the heart of the song's message.
This song is basically a list of instructions for a friend who's about to see your ex. Aitana is asking a friend to pass on a message, but she can't quite decide what that message should be. First, she wants to play it cool. She tells her friend, "Dile que me viste, que le mando un 'Hola'". [Tell him you saw me, that I send a 'Hello']. She wants him to know she's single again but insists, "no me siento sola" [I don't feel lonely]. It's a classic move: trying to seem okay while clearly not being okay.
Then, the real feelings start to spill out. She begs her friend, "Porfa, dile todo lo que estoy sintiendo" [Please, tell him everything I'm feeling], completely contradicting her earlier request to keep it casual. She admits that leaving him was a huge mistake, calling it "una estupidez" [a stupid mistake] and confesses she's "muriendo por hablar con él" [dying to talk to him]. She remembers their time together as "mis mejores años" [my best years] and even apologizes for any songs she wrote about him. It's a messy, relatable message about regret and the lingering feelings for a first love.