Me arrepiento means "I regret". It comes from the important reflexive verb arrepentirse, which is used to express a deep sense of remorse for something you've done wrong.
This phrase is the emotional core of the song. The singer repeats "yo me arrepiento" in the chorus, emphasizing their regret over past actions and their desperate wish to make things right with the person they still love.
In "Quiero Decirte," Spanish pop artist Abraham Mateo teams up with Ana Mena to deliver an upbeat duet all about major post-breakup regret. Both singers are stuck in empty apartments where memories burn like hell ("La casa es como el infierno / Queman los recuerdos"), openly admitting they were masochists for leaving everything exactly as it was ("Y yo en plan masoquista dejé todo tal cual"). Instead of moving on, they spend their days secretly plotting how to win each other back, completely unable to breathe whenever past memories hit them.
The heart of the track is a desperate, straightforward apology: "Quiero decirte que lo siento, que te echo de menos" ("I want to tell you that I'm sorry, that I miss you"). You get some great Spanish conversational phrases here, like "en plan" (used just like "like" or "acting like" in casual speech) and "tirar pa'lante" ("Pa'lante no puedo tirar" meaning "I can't move forward"). Neither person has given up hope ("No pienses que ya te di por perdida" / "Don't think I already gave you up for lost"), turning this duet into a mutual plea for one more chance before it is truly too late.