Rehacer means "to remake" or "to redo." It's an interesting word because it implies rebuilding or starting over after something has been broken or lost.
In the song, the artist sings, "Yo sé que vas a rehacer tu vida, lo único que quiero que tú sepas es que yo no puedo rehacer la mía" (I know you're going to remake your life, the only thing I want you to know is that I can't remake mine). This highlights the pain of moving on after a breakup, where one person is able to rebuild their life while the other feels unable to do so.
Have you ever felt like the sky itself cried when your heart broke? Yiyo Sarante's "Manos De Tijeras" [Scissor Hands] starts right there, with the powerful line "Le dije al cielo que te fuiste y empezó a llorar" [I told the sky you left and it started to cry]. It's a really emotional song about a breakup, looking back at the beginning of a relationship with such fondness: "Tú con el pelo suelto / Y yo con esas ganas de hacerte reír" [You with your hair down / And me with that desire to make you laugh].
But that sweetness quickly turns to pain. He sings, "Me duele, cada que me acuerdo de tus besos / Me duele, porque el tiempo va de ida / Y va sin ruta de regreso" [It hurts, every time I remember your kisses / It hurts, because time goes one way / And has no return route]. He's clearly struggling to move on, asking his ex, "Dime si me echas de menos, tan solo un poquito" [Tell me if you miss me, just a little bit] and admitting, "yo, no puedo respirar, sin ti" [I can't breathe, without you]. It's a raw look at regret, missing someone, and the fear of seeing them with someone new: "en cualquier momento subes una foto / En los brazos de un extraño / Eso sí va a hacerme daño" [any moment you'll upload a photo / In the arms of a stranger / That will really hurt me]. He knows she'll move on, but he can't, saying, "yo no puedo rehacer la mía" [I can't remake mine]. It's a really honest and heartbreaking song about the lasting impact of a lost love.