She was rescued from having to pretend everyone was a failure just because they were different from Mary, and brought into a different world where people could be accepted for who they are.

Mary suggests that the people the main character are trying to impress aren't actual that amazing.

She was giving up on trying to get others to like her, but maybe this would result in more people liking her for who she truly is when she's not pretending.

What does "golliwog" mean? Also, she constantly compares herself to Lonnie, but is there anything she does better than Lonnie?

She didn't do what was best for her, but instead what everyone else did. She was very self-conscious. (indirect characterization)

She was nervous and uncomfortable at school. She thought she was beneath others, so others were insulting and took pride in making her feel worse about herself.

She didn't do many things for herself, but she did more to impress others.

How did she afford this/who would give such a dress to her? Cashmere and pearl buttons were not cheap.

Wouldn't she have kept such a nice dress? Even if she was in hard conditions she probably would have kept the dress and could have sold it.

She was embarrassed of her mother.

She is a teenager, probably in high school.

Her mother didn't think as highly of her as she did of Lonnie, her mother was always comparing them.

Lonnie was vulnerable because she doesn't have a true parent, since her dad ignored her and her mother was dead. However, she was privileged since she was from a wealthy family.

Lonnie was wealthier than her.