A speaker stands alone by the sea at night, and the loneliness is so complete that even the landscape seems to talk back. The sand "speaking my mind" suggests the speaker's feelings have spilled out into the world, so the shore becomes a kind of mirror. Flinging stones at the sea all night reads as restless, a way of throwing grief at something too large to answer. The line about west being "true" with "nobody home" hints at a direction, or a place, that is certain but empty.
The turn comes when the speaker addresses a "valentine" going out to watch the storm "act her part of grief." The storm is treated like a performer, playing the sorrow the speaker already carries. The last line is quietly startling: the person goes looking for grief and meets "who but me." The poem leaves us with a mood of solitude, yearning, and a strange, wry recognition that the grief being sought is already here.