An extended metaphor is a long-lasting metaphor that extends throughout a work.
The sea creates a symbolic boundary for the narrator because she feels isolated in a foreign land. She has gone to the country because it is her husband’s homeland, but it has resulted in a loss of independence. When she reflects on the sand, she does not want to change anything, just as she can change nothing in her life.
I had an idea that the patterns on the stone should be made by nature alone; I did not want one grain of sand, blown by a breeze I could not feel, to change the course because of me.
This does not necessarily mean she has accepted her fate. She is confirming it. In this country, represented by the beach, she is lost. She can only take comfort in the fact that she is part of the larger landscape.