Arrepientas comes from the reflexive verb arrepentirse (to regret or repent) and is the second-person singular present subjunctive form.
Shakira sings "Seguramente con el tiempo te arrepientas" to warn that “with time you might regret it.” The subjunctive mood adds a sense of uncertainty and emotional weight, making this word a vivid, less-common conjugation that learners rarely see in everyday vocabulary but will love mastering through the song.
In "Última," Shakira is laying all her cards on the table for what feels like the very last time. She's telling an ex-partner that it's truly over, even though it hurts. She starts by saying, "Por favor, déjame hablar, no me interrumpas, te lo pido" (Please, let me speak, don't interrupt me, I ask you), making it clear this is her final word on the matter. She acknowledges they were just too different, singing that it was "más fácil era mezclar el agua y el aceite" (easier to mix water and oil). He wanted to go out and take on the world, while she just wanted to be with him.
Even with this finality, she admits it's not easy. She predicts that maybe one day he'll regret it and want to come back, but she also admits she might feel the same way. The core of her pain comes out when she asks, "¿Cómo es que te cansaste de algo tan genuino?" (How did you get tired of something so genuine?). For now, though, she's made a choice. She sings, "He decidido estar sola" (I've decided to be alone), accepting that the only thing left from their time together is the lesson learned.