The speaker of "Flare" has a pretty deep understanding of grief, sorrow, misery—of all sorts of sadness, really. She knows that grief is unavoidable and advises us to let it be our sister. Despite this, or because of it, she urges us to rise up from our sorrow, to turn away from the graveyard where we keep the losses of our past, and be lively and exuberant.
Though our speaker seems to want to tell us that moving on from sorrow can be done rather simply, through determination and paying attention to the natural world, the way she keeps circling and repeating this idea shows that, on some level, she is aware that it is not simple at all.
We don't buy it. The natural world isn't a cure all for sadness. It's merely an escape.