After telling Julien she wants him back, Mathilde asks if he has had sex with Madame de Fervaques yet. He decides not to answer, which just twists the knife in her all the more.
Julien is conflicted. He wants to take Mathilde in his arms, but knows that she'll despise him in only a few days if he declares his love for her.
Julien tells Mathilde straight up that he doesn't trust her. She's too fickle for him and he won't let himself love her anymore. With that, he leaves Mathilde to cry all by her lonesome self.
Julien goes to the opera that night with Madame de Fervaques and cries because he's thinking of Mathilde. Madame thinks that he's just showing his sensitivity as a man because the opera is sad. It makes her like him even more.
Of course, Mathilde is at the opera to see whether Julien is there with Fervaques.