I think Anna's lack of agency and assumed inconsistency may be assigned to her being in a foreign environment and not knowing the "rules". She was pulled out of the country and the environment she was used to, knew and felt comfortable in, to be placed in another country whose language she speaks, but whose conventions she does not know. This makes her self-conscious, insecure and almost paralyzed in her actions. She does not seem to feel, and consequently appear to others, like a “whole” person, as Ethel states: “You’re not all there;” (p.145) Anna does not seem able to make herself "whole", in the sense of bringing her old life along into her new one. Whenever she wants to tell people (e.g. Walter) about her home in the West-Indies they ignore her. And in some situations she even has to disguise that she is a foreigner, because it would cause her disadvantages (Ethel's xenophobia). 

Although I know that these passages can be read differently, I also tend to read her recurring phrase: "I've read books about it" (e.g. p 104) in the way, that due to her foreignness she cannot understand by herself how English people would feel and react and thus has to rely on what others have written about it (which, of course, is hardly ever helpful to her).