Yet ev'n on this her load Misfortune flings, To press the weary minutes' flagging wings: New sorrow rises as the day returns, A sister sickens, or a daughter mourns. Now kindred Merit fills the sable bier, Now lacerated Friendship claims a tear. Year chases year, decay pursues decay, Still drops some joy from with'ring life away; New forms arise, and different views engage, Superfluous lags the vet'ran on the stage, Till pitying Nature signs the last release, And bids afflicted worth retire to peace.
Hey, maybe things are finally picking up for our speaker. Maybe he's finally seeing the silver lining in all of these dark clouds…
No, who were we kidding? Everything is totally terrible all the time, always. Even this peaceful life is plagued by misfortune. With each new day there's a new sorrow: a sister might get sick or a daughter might be sad.
The speaker states that "kindred Merit fills the sable bier." A "bier" refers to a frame on which a coffin is placed at a funeral. So the speaker is figuratively referring to death here. He's suggesting that good things, including "Merit," eventually die. This is one of the difficulties of life: even if it's a good life, we have to watch things (and people we love) die.
The speaker continues to refer to more things that make life full of misery. A friendship gone bad can make us cry. Each new year brings with it decay. Little by little joy drops away from life, which is "withering" or shrinking. Someone cue the sad violins.
New things and new views emerge, leaving us behind, until at last Nature takes pity on us and allows us to die and find peace.