Ni modo is a popular Mexican Spanish phrase that expresses resignation, like saying 'Oh well,' 'What can you do?' or 'There's nothing to be done about it.' It's a must-know expression if you want to sound like a native speaker.
In the song, the singer has been caught cheating and says, "Ni modo que me crea que somos amigos" (There's no way she'll believe we're just friends). He uses the phrase to show he's accepted his fate and there's no use in trying to lie his way out of it.
This song drops you right into the middle of the drama. The singer has just been caught cheating, and he's totally stuck. What can he say? He knows she won't believe any excuses, singing, "Ni modo que me crea que no nos comimos" (No way she'll believe we didn't hook up). But he quickly reveals that their relationship was already falling apart, admitting that giving in to temptation had become normal for him.
Then comes the twist. He claims she's the one who started it all! He says she cheated first, and his actions were pure revenge: "Por eso la engañé yo a ella / Pa' que como me dolió, le duela" (That's why I cheated on her / So that it hurts her like it hurt me). He insists he wasn't always like this, telling her directly, "Yo no era así, así me hiciste" (I wasn't like this, you made me this way). The whole messy situation has left him completely fed up with romance, declaring, "Gracias a ti ya no quiero saber del fucking amor" (Thanks to you, I don't want to know about fucking love anymore).