As an abuse victim, Eveline probably feels degraded. Certainly, she suffers not just at the hands of her father, but also at work. In her present situation, Eveline gets no respect. Eveline hopes by changing her environment to become respected. For, as she sits at the window considering her escape from the tragic situation in which she exists, she imagines that in the new world "she would be treated with respect" and not be demeaned by such as Miss Gavan at the Stores.
Originally, she chose to stay with her father because of a promise to her dying mother to look after the family. When she is ready to go away with Frank, all she can think about is her mother's last words: Derevaun Seraun! Some critics say these words mean "the end of pleasure is pain"; most, however, think it is just gibberish.
Your guess is as good as mine about why she doesn't go with Frank. She thinks about how her mother sacrificed, so does she think she needs to keep sacrificing her happiness? Is she afraid that happiness is unattainable? Is she afraid of the unknown? All of these possibilities could be given to explain her actions.