As she sits looking out of the window, it is clear that Eveline is experiencing a massive internal conflict as she contemplates leaving her life and home and staying in the same stiflying, claustraphobic atmosphere that has so little to offer her. On the one hand, we are told of the conditions of her life that she has to cope with: a job where she is bothered by her superior and a father whose tyranny makes her life very hard. On the other hand, this is her life, and there is comfort in familiarity and in what you know. Perhaps her conflict is best expressed in the following sentence quote:
She had hard work to keep the house together and to see that the two young children who had been left to her charge went to school regularly and got their meals regularly. It was hard work--a hard life--but now that she was about to leave it she did not find it a wholly undesirable life.
However, it is the memory of her mother's death that gives her the impetus to stand up and leave her home, desiring escape above all else and Frank to "take her in his arms, fold her in his arms" and save her.
I think therefore we can conclude that Eveline's sudden decision at the end to stay is as a surprise to herself. She had planned on going and had organised everything to leave, and finally meets Frank at the harbour to get the boat. If her thinking had been governed by wise caution, she wouldn't have gone to all that trouble. She clearly did not know until the end what it was that she was going to do, and her final decision is a response to the feeling of "All the seas of the world" tumbling about her heart. It is an instinctive response to forces bigger than her that cause her to stay.