Why did Angela stop wearing her wedding ring?

Angela is engaged to Dr. Deere. Everyone congratulates her, but we soon see that this is all anyone talks about with her. She seems to be defined by her engagement to the doctor.

She was not fine. Why did they ask about Denton all the time, as though she was nobody without him?

In chapter nine, Angela tells her mother that she stopped wearing her engagement ring due to a rash on her finger. However, as the story goes on, we begin to suspect there's another reason.
In chapter thirteen, Angela confides to Theo that she once dreamed of attending medical school and becoming a doctor. In chapter fourteen, Angela and Denton argue, and he questions whether she wants to get married at all.

Angela twisted the engagement ring her mother made her wear in spite of the rash. No, she did not want to get married, not right away, but she couldn't say it, she couldn't tell him—them, not like that. Denton would be so hurt, her mother . . . the engagement was announced in the newspaper, the wedding gown, the shower . . . but once they found out she wasn't their perfect Angela . . .

In chapter sixteen, we find out that Angela is the bomber and set a bomb at her own bridal shower, leaving her burned and scarred.
After the game is over, Angela breaks up with Denton and goes back to school. Year later, in med school, they meet again and eventually get married. This shows us that Angela loves Denton but just wasn't ready to get married yet.