Cabrón is a very strong, colloquial Spanish word that can mean "asshole" or "bastard" depending on the context. While it's often used as an insult, it can also be used to describe something as incredibly difficult or intense.
In this song, the lyric "Fue el error más cabrón que jamás cometí" translates to "It was the most fucked up/biggest asshole mistake I ever made." Here, it emphasizes the extreme regret and self-blame the singer feels, making it a powerful and memorable word choice that stands out from typical romantic song vocabulary.
"Abril" is Reik at their most regretful: a guy replaying the breakup he caused and hating himself for it. He doesn't dress it up. "Fue el error más cabrón que jamás cometí" [It was the most f*cked up mistake that I ever made]. That's not poetic distance, that's just admitting it. The trigger is the season itself: "Otra vez se acerca abril" [April's getting close again], because it brings the memory back whether he wants it or not, "Y con su olor regresa a mí la imagen" [And with its scent the image comes back to me].
What makes it land is how plainly he asks for a second chance. No metaphors, just "Quédate conmigo" [Stay with me], repeated four times like he's not convinced it worked the first three. And the hook isn't really about the person anymore, it's about wanting the month itself to give him another shot: "Que vuelva abril, regresa abril" [That April comes back, come back April], as if reversing the season could reverse the mistake.