Arrepentido translates to "regretful" or "repentant". It's a powerful adjective that expresses a feeling much deeper than just being sad (triste).
In the song, Carin León sings, "Estoy arrepentido, enfermo, triste y solo" (I am regretful, sick, sad, and alone). This one word reveals that he recognizes he made a mistake in the past, and this profound sense of regret is what drives him to plead for a second chance.
Carin León puts all his cards on the table here, and honestly, he sounds miserable. The title says it all: "Quisiera saber" ("I would like to know"). He spends the whole track obsessing over whether an ex still thinks about him ("Si todavía me quieres, si todavía me extrañas" ["If you still love me, if you still miss me"]). There is zero pride left. Instead of pretending he moved on, he straight-up admits that his attempt to forget her completely failed: "Te quise olvidar, pero no pude lograrlo" ("I tried to forget you, but I couldn't pull it off").
Things get dramatic fast when he claims he is "enfermo, triste y solo" ("sick, sad, and alone"). Calling himself "enfermo" (sick) is not about catching a cold; it is the Spanish way of saying heartbreak has worn down his physical health. He even refers to his love as "mi pobre corazón" ("my poor heart"), begging for forgiveness and asking to rekindle "aquel amor bonito" ("that pretty, gentle love"). It wraps up with the ultimate plea: "Sólo tú puedes darme la vida otra vez" ("Only you can give me life again"). It is needy, dramatic Regional Mexican longing at its best.