If this is the excerpt:
Heard melodies are sweet, but those unheard
Are sweeter; therefore, ye soft pipes, play on;
Not to the sensual ear, but, more endear'd,
Pipe to the spirit ditties of no tone:
Fair youth, beneath the trees, thou canst not leave
Thy song, nor ever can those trees be bare;
Bold Lover, never, never canst thou kiss,
Though winning near the goal yet, do not grieve;
She cannot fade, though thou hast not thy bliss,
For ever wilt thou love, and she be fair!
Then the part that reflect the theme that art is immortal is
Though winning near the goal yet, do not grieve;
She cannot fade, though thou hast not thy bliss,
It implies that there is no need for grieving because She cannot fade. She will last for a long time.