Arrepiento comes from the reflexive verb arrepentirse, meaning 'to regret'. It's a powerful word used to express deep remorse for a past action or inaction.
In the song, David Bisbal sings, "Te lo juro que ahora me arrepiento" (I swear that now I regret it). He is filled with regret for not saying "I love you" in time, and this intense feeling is the driving force behind his plea for his love to come back.
Ever told a mutual friend that you are completely over your ex, only to go home and obsess over them all night? That exact lie drives this duet between Spain's David Bisbal and Mexican singer Danna Paola. They admit that swearing off each other back in April was completely fake: "Fue mentira lo que le dije a tu amiga / Que ya no te extraño / Porque sí te extraño" ("It was a lie, what I told your friend / That I don't miss you anymore / Because I really do miss you"). Instead of moving on, they are stuck in regret, admitting "Te pensé el invierno entero / Y nunca dije que te quiero a tiempo" ("I thought of you all winter / And I never said I love you in time"). Without their partner around, life falls apart completely—or as the chorus bluntly confesses, "sin ti soy un desastre" ("without you, I am a disaster").
There is a great Spanish idiom here that reveals how stubborn the singers really are. They sing, "no vale un punto y aparte / Cuando se trata de ti" (literally, "a new paragraph isn't valid / when it comes to you"). In punctuation, a punto y aparte is a full stop that ends a paragraph and moves down to start a fresh topic. By refusing to use one, they are saying their romance cannot just turn the page or start a new chapter. Instead, they tell each other "Vas a quedarte sin pretextos para huir" ("You're going to run out of excuses to run away") and beg for a second chance before it is too late.