The disillusionment phase in Nora's life occurs as a result of several things.
In different conversations with Krogstad, Mrs. Linde, and even Dr. Rank, Nora shares some moments in her life that have affected her in some way. She talks about the risks she has taken, from asking Krogstad for a loan, to forging her father's signature to get the loan. She also lets the audience into other intimacies: she tells Linde how she hides the information from Torvald, and also lets Linde in how the dynamics of her marriage are. It is Linde who first understands that Nora's marriage is basically a staged one.
This means that Nora is indeed aware that her life is far from perfect and that she hangs by a thread should her husband ever find out about what she did. Yet, we can also sense that Nora still holds a small amount of hope: that, if things were to be exactly as she would wish, Helmer would appreciate the sacrifice behind her actions.
When Torvald is no longer as devoted to me as he is now; when my dancing and dressing-up and reciting have palled on him; then it may be a good thing to have something in reserve —[Breaking off.] What nonsense! That time will never come.
But the time did come. In fact, Helmer's first reaction as he reads Krogstad's letter he does not consider that Nora's indiscretion was made out of need, and not out of want. In fact, his reputation is what he actually cares about more than anything. Then came a series of cruel insults and reproaches that would have shocked any wife, especially Nora who, deep inside, would have never imagined that Helmer would go against her in this matter. When all of this happens Nora's period of disillusionment begins.
It first starts with the shock of hearing Helmer say those words to her. Then, when Helmer tries to make up to her after he realizes that there is no real issue, he has no idea what harm he has caused in Nora; her ego is forever bruised, her trust in her husband is over and, perhaps, even the love- or what she thought was her love for him.
Indeed, you were perfectly right. I am not fit for the task. There is another task I must undertake first. I must try and educate myself—you are not the man to help me in that. I must do that for myself. And that is why I am going to leave you now.
Here, Nora comes to the realization that she has been basically used; first by her father, as a plaything and now also as Helmer's own plaything. She now knows that, perhaps, her life has been wasted; it is best to leave behind what cannot be undone and move on to try and start over again.