Tormento translates to "torment" or "anguish". It's a strong, evocative word that captures a deep sense of suffering or distress.
In the song, the artist sings "Non sempre tu che invece dai tormento" (Not always you who instead gives torment). He uses this word to describe the emotional pain and distress caused by a past relationship or a lingering sadness, contrasting it with the new, fresh air he now breathes. It's a powerful way to express overcoming a difficult emotional state.
Grignani spends this whole song breaking up with his own bad mood. The person he's leaving isn't a lover, it's tristezza, sadness itself, personified as a woman who keeps him up all night. She 'talks like an actress' who'll eventually cast him in her movie, and he pictures her 'watching me from the corner of the bed' like she owns the room. The chorus is him finally standing up to her: 'Ciao e arrivederci' ('Hello and goodbye'), followed by 'Tristezza mia, tu resta, io vado via' ('My sadness, you stay, I'm leaving').
By the last stretch of the song he's stopped arguing and is just repeating 'Io vado via' ('I'm leaving') until the words do the work themselves. It plays less like a farewell to a person and more like a pep talk set to a big rock chorus. Name the thing that's dragging you down, then walk out on it.