The turning moment for Nora in Act 3 was her husband's response to Krogstad's letter revealing that Nora had forged a signature on a loan. Nora had expected him to realize that she had forged the loan as a sacrifice for his sake; she expected him to realize how much she had loved him and to return that love. She had also expected him to allow his own reputation to fall for her sake. She, however, was expecting to spare his reputation by committing suicide. It is Torvald's sacrifice and her reciprocal sacrifice through suicide that she refers to as the "wonderful" and "terrible" thing that is "going to happen" (II). We especially see her expectations of sacrifice and suicide revealed in some of her lines in Act 3, such as, "You shan't save me, Torvald!" followed by, "Let me go. You shall not suffer for my sake. You shall not take it upon yourself" (III).
Instead, contrary to her expectations, Torvald thinks only of his reputation and nothing of how Nora made a sacrifice for his sake when she took out the loan to save his life. It especially begins to dawn on Nora that he is thinking only of his own reputation and not at all willing to sacrifice for her in return when he demands of her:
Do you understand what you have done? Answer me? Do you understand what you have done? (III)
Those three little questions are really the turning point in the dialogue. It is after these lines are delivered that Ibsen describes Nora's face as having a "growing look of coldness," as she says in reply, "Yes, now I am beginning to understand thoroughly. It is after this moment that Nora realizes how thoroughly naive she is about the world, decides she is not fit to be a mother, and decides she must leave to educate herself.