Yeats addresses two different contrasts in his poem "The Wild Swans at Coole". The first is between himself in the present, and when he first wlaked along the water where the swans are found. The second is between himself and the swans.
The poet says, "The nineteenth autumn has come upon me since I first made my count", meaning that nineteen years have passed since he first came to see the swans. He realizes that "All's changed since I, hearing at twilight, the first time on this shore, the bell-beat of their wings above my head". The change is within himself - he was "unwearied" back then, and "trod with a lighter tread", but now his "heart is sore", and he is tired.
Yeats then compares his condition now with that of the swans. While he himself is old and weary, the swans are not. They are still "brilliant creatures", and as "they paddle in the cold companionable streams or climb the air, their hearts have not grown old". The swans still retain a sense of "passion or conquest", and are "mysterious" and "beautiful", capable still of "delight(ing) men's eyes". The poet, in contrast, feels that all these wonderful attributes no longer apply to himself; he is tired, and treads heavily now in body and soul.