I think the turn happens in the middle of the poem. Before the speaker was talking about things he would lose and things he wish he could of done before death. The last lines of the poem instead of listing these things, instead talk about wanting recognition before death. This is then elaborated on in the last few lines of the poem.Never have relish in the faery powerTill love and fame to nothingness do sink.
The speaker is afraid of losing love after dying. The speaker also fears dying with recognition for the things they did do and so dying without anyone knowing their accomplishments.
And when I feel, fair creature of an hour,That I shall never look upon thee more,Never have relish in the faery powerOf unreflecting love;--then on the shore
Of the wide world I stand alone, and thinkTill love and fame to nothingness do sink.
These last few lines are about how the speaker feels they will be nothing after death. The speaker fears that after death there will be nothing left of them in the world and they will become forgotten. The speaker has recurring themes of not wanting to become forgotten without recognition. The urge to be known after death is also a theme recurring. The speaker wishes to continue being known for their accomplishments after death and fears they have not done enough accomplishments.