The burdensome mother
The mother is a potent symbol for everything Rachel dislikes about her life. Instead of being launched into an adventurous life, she is anchored into a town with ideas about womanhood that are burdensome to her. She feels lonely and like she is wasting her time. The mother is literally wasting away, and she is a symbol for the attachment Rachel feels to her family duty. By freeing herself from the emotional burdens of her mother, she could be free to create a new self.
Nick, falling in love
By falling in love with Nick, Rachel participates in an instinctual process that he knows could lead to her being rescued from the life she resents. Plus, Nick could help her departure look less voluntary, because what mother would stop her own daughter from falling in love? The summer is full of passion, and Rachel initiates much of the romance, but Nick loves the attention. She doesn't notice that the odds of Nick taking the relationship seriously are thin. He represents the hope she needs to have for a better future.
The town
Rachel's relationship to her community is symbolic. At first glance is seems self-explanatory, but Rachel's struggle to relate to others symbolizes the conflict between all individuals and groups. Also, their opinion of her is shaped by opinions about gender that she doesn't share, so in order for her to leave, she will have to break out of their socially imposed constructs. They symbolize the traditional influence of community.
Sickness and death
Although her mother's sickness and death mean one thing to the mother, they mean something completely different to Rachel. Rachel is also on a fixed schedule. She doesn't have forever to live, and she's already nearing some important biological deadlines. If she is going to be a mother herself, she needs to get going, and she knows it, so secretly, she sort of wants her mother to die, so she can live. The symbolism is layered, but ultimately, it is attachment to her mother that Rachel resents.
The decision
Instead of waiting around for her mother to die, she just leaves. This is a painfully symbolic reminder that she was always free to leave. She knew she didn't agree with the political opinions of certain folks in town, but she didn't realize she was perfectly free to leave without guilt or shame. Her guilt and shame were the real boundaries she couldn't cross, so the decision to leave is the voluntary acceptance of responsibility. She chooses to be responsible for herself instead of pretending her mother needs her to stay behind.
