I think you have to tie it to the previous line -- the lesson comes too late, and it's made of sand, i.e. it can never last. It suggests to me that we don't learn what we need to from love relationships -- or at least that the songwriter doesn't think he did in this instance. Part of the whole tenor of the song is along these lines: "I know the whole thing was my fault, I know I'm impossible -- please don't go, please don't go." I think it's very poignant, a beautiful song -- as Tom Paxton tends to turn out.