Bregaste comes from the verb bregar, which means to struggle, to work hard, or to deal with something. It's a versatile word often used in Caribbean Spanish.
In the context of the song, the line "Aunque bregaste mal" translates to "Even though you handled things badly" or "Even though you dealt with things poorly." It implies that the ex-partner acted in a way that caused problems or was unfair, making it an interesting and somewhat unique word choice for a breakup song.
"Me Llora" is about moving on after a breakup and not looking back. The singer admits the relationship had good moments, but their ex also treated them badly, so they weighed the two sides and decided it wasn't worth staying.
Now the ex wants them back and is the one crying over the breakup, but the singer refuses to even glance in that direction. They've replaced the old relationship with someone new and are focused on enjoying life, no longer bothered by jealousy or by what their ex is doing.