Most love songs are written for somebody else. El Chojin flips that: this one is a love song he writes to himself. He even admits it up front: "He escuchado mil canciones de amor preciosas, dedicadas todas a otras personas" (I've heard a thousand beautiful love songs, all dedicated to other people), so "me escribí una canción de amor a mí" (I wrote myself a love song).
The best part is how honest he is about why. He calls himself "esa persona que no me va a dejar" (that person who is never going to leave me), the one he has "tratado tan mal" (treated so badly). It is not ego, he insists: "debo cuidar de mí si quiero cuidar bien del resto" (I have to look after myself if I want to look after everyone else properly). There is a funny, very modern moment too, where he offers to send you a link to a playlist of songs about you, the way he has left a thousand hearts on your profile.
The chorus is pure advice from his parents: "Aprende a quererte, corazón" (learn to love yourself, sweetheart). Corazón literally means heart, but here it is a pet name. Then come the gentle orders: "Sé amable y trátate bien" (be kind and treat yourself well), "Relaja tu mente y tu cuerpo también" (relax your mind and your body too). For a learner it is a goldmine of reflexive verbs like quererte and trátate, where the action points back at yourself, which is exactly the point of the song.
The second verse goes somewhere lonelier: a "cena pa' uno" (dinner for one), a couple of beers with instant ramen, and the idea that to forgive yourself you need to forgive others. Pa' is just a sung, shortened para (for).