In The Hate U Give, what does it mean when Starr refers to different versions of herself? Why does she need to be a different version of Starr in different settings, and why does she say "I have to keep them separate"?

Starr states numerous times throughout the text that she has one version of herself when she is at home in Garden Heights and another when she is at school at Williamson Prep.
The reason for this is because Starr lives in an all-black neighborhood, yet she attends a nearly all-white private school. Starr feels as though she has to keep these two identities separate in order to fit in with the people with whom she associates in either location.
For instance, Starr learns early in life that her friends at Williamson Prep do not fully accept her Garden Heights origins. Her friend Hailey was not allowed to go to Starr's house for a sleepover, because her parents said it was in the "ghetto."
Likewise, Starr doesn't feel comfortable sharing her Williamson self in Garden Heights. One detail that demonstrates this is that she is afraid to tell her father that her boyfriend Chris is white. Starr doesn't want to disappoint her father—who is a devout believer in black empowerment philosophy—by telling him she chose to be involved with a white boy at Williamson Prep.
Overall, Starr keeps these identities separate because she wants to be accepted in each community, and she fears that integrating her two selves will draw ire from those she wishes to accept her.