It would appear that Susan, the main character in the story, began to lose her mind after she had children, and began to really consider why she married and chose this (the married) lifestyle. The author introduced the concept by describing their marriage as
Their life seemed to be like a snake biting its tail.
The idea of their marriage was described as a logical matter. However, it was the logical nature of the marriage, and not fully understanding why they were both invested in the marriage, that made Susan start to unravel in her sanity. This notion of the logical marriage appears again when Susan's husband admits to cheating on her. On page 528, Susan explains that Myra Jenkins is no one and should have no significance in their lives. However, Susan cannot shake the feeling of betrayal that she has inside.
So either the ten years' fidelity was not important, or she isn't (pg. 528).
So it would seem that Susan's inability to align her feelings with her intelligence caused her to lose her mind. She continued to feel bitterness towards her husband, disgust towards her children and life, and yet, logically her brain said they were happy, affairs didn't bother her or matter, and they could continue living this life. She could not find a logical reason to feel anything. She could find no logical reasons to cry. As the years moved on, Susan continued to despise what she had become, what she was feeling and how no one understood their happiness was anything but.
Shortly after Susan brought her twins to their first day of school, she succumbed to the insanity. Susan unwillingly returned home and sat in the garden where she felt like
...like a panic: as if an enemy was in the garden with her. (pg. 530)
It was at this point that she realized she lost herself while living this "happy" and logical life. She explained that she went from being married, to being a mother, to not understanding who she was, alone. She felt things that she never really allowed herself to feel. The feelings, then, consumed her and made her think more and more of them. She was guilty for feeling. She felt bounded by her life, and trapped. She was longing to be free.
No, her state (whatever it was) was irrelevant, nothing to do with her real good life with her family. She has to accept the fact that, after all, she was an irrational person and to live with it. Some people had to live with crippled arms, or stammers, or being deaf. She would have to live knowing she was subject to a state of mind she could not own (pg. 534).
And with that notion, Susan continued to fall deeper and deeper into depression. However, she was not able to identify depression as a thing. She continued to deny herself feelings that her logical mind considered irrational. She continued to struggle in silence, with feeling bound; until she killed herself.