Me la rifé is a powerful Mexican slang phrase meaning 'I gave it my all' or 'I went all out'. While the verb rifar literally means 'to raffle', this expression is used to say you risked everything for someone or something.
In the song, Junior H sings, "Si contigo siempre me la rifé" (Since with you, I always gave it my all), to express the deep pain of being betrayed after putting his whole heart on the line. It's a key phrase for understanding the raw, emotional language of modern Regional Mexican music.
Junior H didn't hear it from her. He went looking for a reason and got it from "lo que me decía la gente" (what people were telling me), which is a humiliating way to find out. He's drinking through it too: "me trae bien loco la Champán Rose" (the rosé champagne has me going crazy). For a breakup song, that's a pretty honest picture of how badly he's handling it.
Then the hurt turns sarcastic. The chorus keeps bending the word mentir (to lie): "miénteme de nuevo" (lie to me again), "miéntete a ti misma" (lie to yourself), and finally "miéntele y úsalo" (lie to him and use him), aimed at the new guy. "Me la rifé" is Mexican slang for going all in or risking everything for someone, so "si contigo siempre me la rifé" (if I always went all in for you) is the line that stings most. He also calls her "mija", a shortened "mi hija" (literally "my daughter") that Mexicans use like "girl", and here it sounds condescending on purpose. His parting shot, "al final sola te vas a quedar, sola y nomás" (in the end you'll be left alone, just alone), is petty. That's also exactly why it works.