Atormentar means 'to torment' or 'to haunt'. It's a strong verb that conveys a sense of persistent distress or suffering.
In the song, Marília Mendonça uses it to describe how past memories "atormentar minha cabeça" (torment my head), highlighting the painful and inescapable nature of her recollections of a lost love. This word perfectly captures the emotional intensity of the lyrics.
Maiara & Maraísa bring in Marília Mendonça for a live sertanejo duet about a breakup neither woman has moved past. "Você foi embora / E deixou uma saudade e com ela a solidão" ("You left / and left behind a longing, and with it the loneliness") sets up the ache: someone walked out, and the person left behind is drowning in saudade — that specifically Brazilian, bittersweet kind of longing.
The chorus turns the hurt into a confession: "Só pensando em você / É inútil tentar te esquecer" ("Only thinking about you / it's useless trying to forget you"). She can pretend she's over it — "Posso até disfarçar / Que deixei de te amar" ("I can even pretend / that I stopped loving you") — but the tears keep giving her away.