Arrepentido means deeply regretful or repentant, and it's packed with intense emotion. It's a more powerful and specific term than the common lo siento, as it conveys a profound sense of remorse and guilt for a past action.
In this song, the singer repeatedly emphasizes "Estoy tan arrepentido" (I am so regretful) for letting his pride and cowardice ruin the relationship. This word is the emotional core of the entire song, capturing the painful realization of having made an irreversible mistake, making it a very compelling word to learn.
Have you ever messed something up so badly that you wish you could turn back time? That's exactly where Morat is in this song. The singer is filled with regret over a breakup that happened three months ago, and he's finally admitting it was all his fault. He calls himself a "cobarde que por orgullo no está contigo" (a coward who isn't with you because of pride). He confesses that even though she cried first, neither of them really wanted to break up: "Yo no quería despedirme / Y tú no querías olvidarme" (I didn't want to say goodbye / And you didn't want to forget me).
Now, he's desperately hoping it's not too late. He imagines that maybe she's starting to forget him, that their story "ya no cuelga de un tal vez" (no longer hangs on a maybe). But he's still holding out hope, dreaming of a future where he won't have "raspadas las rodillas por rogar ante tus pies" (scraped knees from begging at your feet). His final plea is simple and powerful: if she just dares to come back, she'll realize he never, ever forgot her.