The first two sentences of the poem tell you the theme:All the new thinking is about loss.In this it resembles all the old thinking.
And the poet is recalling his past experience with "her".He remembers many particulars,but as he writes:each particular erases the luminous clarity of a general idea.
So in the poem we can find many descriptions of particular things,but the poet himself is confused.He cannot tell why it has come down to all this in a clear way,but the lost feeling is reminding him again and again:Such tenderness,those afternoons and evenings,saying blackberry,blackberry,blackberry.
Hence we find the poet is expressing something vague by telling us something specific.Isn't life like this:we know every detail,but we cannot grasp it as a whole,let alone its rules or siginifcance?