This is a breakup song told from the side of the person who is done. It opens flat and final: "Ya no quiero saber nada del amor" (I don't want to know anything about love anymore), and then the title line lands, "Hoy, hoy decidí dejarte" (Today, today I decided to leave you).
The other person wants to keep going, but the singer calls the whole relationship a show that has stopped selling: "Ya fue mucho teatro y creo que la función ya no se vende tanto" (It's been too much theater and I think the show doesn't sell much anymore). That theater image is the best part of the song, and it's a little cruel. Then comes the blunt one, "Todo se fue a la mierda" (Everything went to sh*t), which is a common, very rude Spanish way of saying it all fell apart.
The twist is that she used to ask him what he felt, and only now that he's leaving does she want to see him. His answer: "Creo que ya es muy tarde" (I think it's already too late). Even so he admits "aunque duele es lo mejor para los dos" (even though it hurts, it's best for both of us).
Listen for the spoken shout-out "Nomás no se me agüite": agüitarse is Mexican slang for getting sad or bummed out, so it means "just don't get down on me", aimed at the two singers themselves.