Regaste comes from the expression 'regarla', a common slang term meaning 'you messed up' or 'you blew it'. Its literal root, the verb regar, means 'to water' (like a plant) or 'to spill'.
In "0 SENTIMIENTOS," the singer laments, 'y aún sabiendo que tú la regaste' (and even knowing that you messed up), calling out his ex for her mistakes. This is a fantastic and very common piece of slang that will make you sound more like a native speaker.
This song is a brutally honest story about getting cheated on and feeling completely numb afterwards. The singer kicks things off by admitting it was his mistake, or "Mala mía" [My bad], for falling for someone who was beautiful on the outside but whose heart didn't work. He lists all the things he wasted on her, like a Birkin bag and vacations, only for her to treat him badly.
The chorus is where it gets really direct. He says because of her, "traigo cero sentimientos" [I have zero feelings]. He's so emotionally shut down that when he sends a text saying "TQM" [I love you a lot], he doesn't feel a thing. The moment that broke him? Seeing her in a club kissing the guy she said was just a friend. He flips the script with a very blunt line: he might not feel the love, but when he sends a "CTM" [a very strong insult], he definitely feels that. It's a classic heartbreak song with a tough, modern edge, ending with the ultimate boast: there are a hundred girls like her, but she'll never find another guy like him.