"Mejor Que Tú" (Better Than You) is a breakup song sung from the other side of the breakup: she got through it, and she wants her ex to see it. It opens with a command, "Mírame ahora" (Look at me now), followed by "Ya no estoy sola" (I'm not alone anymore) and "Tu deseo no se cumplió" (Your wish didn't come true). He wanted her to fall apart. She didn't.
The verses list what he did: he left, then kept blaming her and saying "que no soy nadie" (that I'm nobody). Her answer is pointed. "Sabrás que no eres indispensable" (You'll find out you're not indispensable), and the knockout comes at the end of every chorus: "Para mi buena suerte, él es mejor que tú" (Lucky for me, he's better than you). She even calls him "el más cobarde" (the biggest coward).
A useful word here is coraje. In Mexican Spanish it often means anger or rage rather than courage, so the chorus line about putting up with all her coraje "porque te amé" (because I loved you) is about swallowing a lot of anger for love. Also watch "Cuida muy bien tus palabras" (Watch your words carefully): cuidar means to take care of, and here it works as a warning.