Estallar means 'to burst' or 'to explode'. It's a strong, evocative verb that suggests a sudden, intense release of something.
In the context of Carla Morrison's song, the lyric "Mi locura está por estallar" (My madness is about to burst/explode) vividly conveys a feeling of being overwhelmed and on the verge of losing control. It's a powerful image that captures the emotional intensity of the song.
This song is a beautiful mess of feelings for someone you probably shouldn't be with. Carla Morrison swings from feeling happy to feeling bad, but ultimately says to her lover, "daño es lo que me haces más" [what you do most is hurt me]. It's a relationship so intense that she feels her "locura está por estallar" [madness is about to explode]. The whole song is basically a plea for this person with pale blue eyes to stay, even though it's a painful situation.
The lyrics are full of regret and impossible wishes. She sings about having this person as her everything, but admits she didn't know how to keep them: "Que tuve sin saber conservar" [That I had without knowing how to keep]. The relationship is also a secret, or at least forbidden. She knows it's wrong, singing, "El hecho que eres de otro / Me recuerda que sigo sin ver / Que es pecado amarte, ya lo se" [The fact that you belong to someone else / reminds me that I still don't see / that it's a sin to love you, I already know]. But even knowing all that, she'd do it all again.