The answer to this question lies in the way that Eveline is shown to experience a sudden, shocking epiphany at the end of the tale as she is about to board the ship that will take her away with Frank to start her new life and give her the escape that she so desires. We are told that Eveline feels a bell, clanging upon her heart, and then note the following description of what she experiences:
All the seas of the world tumbled about her heart. He was drawing her into them: he would drown her. She gripped with both hands at the iron railing.
The image is one of drowning as Eveline is faced with the immensity of what she is doing. Having prayed to God for guidance, she is suddenly overwhelmed by this feeling of what she is metaphorically doing by leaving Ireland, and as a result, Eveline experiences a literal paralysis that becomes a more powerful force than the action that caused her to stand up and leave her home.
I don't actually believe that Eveline knew she was not going to leave Dublin. The careful planning and arrangements, and her action in leaving the house after remembering her mother's fate indicates that Eveline at least entertained the thought of going. It is only when she has this epiphany of what the act of leaving will actually mean that she is struck with paralysis and realises that duty is more important than her escape.