What is Bertha frustrated about?

Bertha has been living most of her life in a dream world, where, to her mind, everything in her life is lovely and perfect. On the evening this story takes place, she is having a dinner party, and everything seems more perfect than ever. She loves her beautiful house, loves arranging her fruit, loves her baby, and loves the beautiful pear tree outside the window.
During the evening, one last obstacle to her perfect happiness falls away:

For the first time in her life Bertha Young desired her husband. She'd been in love with him, of course, in every other way, but just not in that way.

She now sexually desires her husband.
She is frustrated, however, at the end of the story, when she goes out into the corridor where her husband is saying goodnight to Miss Fulton, because she realizes they are having an affair. His lips form the words "I adore you" to Miss Fulton, and she touches his cheek tenderly with her fingers. He whispers "tomorrow" to her.
Bertha recognizes that her "bliss" is hardly perfect. The title of the story is, in fact, ironic.