A reasonable inference that one can make out of this poem is that you should not be worried about your fate, because at the end people are part of a generation which has its own struggles and at the end, people have to face their own destiny,being courageous, as the previous generations were and have been.
This is the reason why is possible to conclude that the two lines best linked to the theme are:
"Be comforted; the world is very old,
And generations pass, as they have passed"
