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.
If you have ever been played by someone you showered with luxury, "0 SENTIMIENTOS" will hit home immediately. Grupo Frontera and Fuerza Regida team up to tell a classic tale of regret and betrayal. The singer admits his mistake right off the bat with "Mala mía, por fijarme en quien yo no debía" [My bad, for looking at someone I shouldn't have]. He bought her an expensive Birkin bag ("La bolsa Birkin que te regalé, ¿pa' qué?" [The Birkin bag I gave you, what for?]) and paid for her vacations, only to watch her make out with another guy at the club ("comiéndose a besos" [eating each other up with kisses]). To make matters worse, that guy was supposedly "sólo era tu amigo" [just your friend].
Now, his heart is completely frozen over. He has turned totally cold, declaring "traigo cero sentimientos" [I'm carrying zero feelings]. There is some brilliant texting slang here that shows just how bitter he feels: he says that when he texts "TQM" (short for te quiero mucho, or "I love you so much"), he feels nothing at all. But when he sends a "CTM" (an abbreviation for the heavy Mexican insult chinga tu madre, meaning "go screw yourself"), he means every single letter. She took him for a fool ("Me agarraste de pendejo"), but he gets the last word by reminding her that while there are a hundred girls like her, she will never find another man like him.