What was one reason Mrs. Pontellier was unhappy in The Awakening?

Let us remember what happens to Edna Pontellier during her
time on the beach. She experiences a kind of "awakening" that makes her aware of the
emptiness of her own life and marriage and also give rise to a desperate desire to be
loved and to be held that she believes can be satisfied through her relationship with
Robert. However, as Chapter Fifteen makes clear, it is ironic that having undergone this
internal transformation, Edna should have the means of gaining what she feels she
desires snatched away from her so cruelly through Robert's voyage to Mexico and away
from her. Note how her thoughts are described at the end of this
chapter:

The
present alone was significant; was hers, to torture her as it was doing then with the
biting conviction that she had lost that which she had held, that she had been denied
that which her impassioned, newly awakened being
demanded.

Thus we can see
that one reason for her unhappiness comes from her new awakening, with its new desires
and wants, and the removal from her of the person who can satisfy those desires and
wants. This results in Edna being left a deeply unsatisfied and unhappy
woman.