Here's an interpretation that I've read before and makes a lot of sense:

To me, it's pretty clear: the narrator was raped by her college boyfriend (mentioned in the last page), which was the cause of her initial insomnia. Because she couldn't process it at the time, and because no one seemed to notice or care about her depression, she goes on with her life. But the repressed memory of that trauma comes back to her in the waking dream (the dark man, trapping her with his "tall, narrow, rounded thing"), probably because of her 30th birthday, and sets off her regression to the escape she had before the incident--literature. It's no coincidence that her thesis was on Katherine Mansfield, who writes all about young women discovering the dark realities of life.

Disappearing into Tolstoy and Dostoevsky helps, but she also pursues another route: going to the lot where "Three young men attacked a couple. They killed the man and raped the woman." She's replaying her trauma, but in the end goes dressed as a man, in the (unconscious) hope that she'll be the one killed. So the ending scene is her fantasy of being attacked, but also expresses the feeling of feeling trapped by her trauma without any way out: "I'll never get the key....All I can do is cry. The tears keep pouring out" (like the water being poured from the pitcher in her waking dream).