Torvald has shown Nora his true colors, and, once she's seen who he truly is, she cannot unsee it and she cannot forgive. When he learned of the loan Nora took out from Krogstad, he is horrified, and he says all manner of incredibly unkind things to her, despite the fact that she took out the loan and has diligently paid it back, with interest, in order to afford the trip abroad that saved Torvald's very life. He says to her, in part,
As for you and me, we must make no outward change in our way of life—no outward change, you understand. Of course, you will continue to live here. But the children cannot be left in your care. I dare not trust them to you.—Oh, to have to say this to one I have loved so tenderly—whom I still—! But that must be a thing of the past. Henceforward there can be no question of happiness, but merely of saving the ruins, the shreds, the show—
In other words, then, he expects her to go on living in their home although he cannot see how they will ever be happy together again, and he refuses to allow her to continue caring for their children. Torvald says that the best they can do now is to save "the show"—implying that the most important thing about their marriage is the way other people see it, the way other people see them. He never considers Nora's feelings. He even insults and berates her, calling her "a hypocrite, a liar—[and] worse, worse—a criminal." He's refused to allow her to explain herself, shouting, "be silent!" when she attempts to do so. He accuses her of having "no religion, no morality, no sense of duty," all of which she has inherited from her unprincipled father; in short, Torvald now sees her as "an unprincipled woman" who has brought "disaster and ruin" on him in repayment for the eight years in which he did nothing but "pet and spoil" her. Nora's actions may have been misguided, and she may have behaved unethically, but she did so with good and loving intentions, intending to hurt no one; in fact, she did it in order to save Torvald from an early death. When she fails to act like a "doll" for once, he turns on her, cruelly. His words make it evident that he does not truly love her, that their relationship has mostly been about "the show," and she cannot forgive him for this.