“No, the heart that has truly lov’d never forgets,
But as truly loves on to the close;
As the sunflower turns on her god, when he sets,
The same look which she turn’d when he rose.”

Irish Melodies, Believe Me, If All Those Endearing Young Charms, by Thomas Moore [1780-1852].

Quotation from Bartlett’s Familiar Quotations by John Bartlett.
Published by Little, Brown and Company. 1955. Page 439.

To read more about Thomas Moore and this poem: Poetry Foundation.