The speaker in Sonnet 131 is lamenting the fact that his love is not returned, but simultaneously saying that he's triumphing over the situation. The basic message is that one can see or interpret beauty even in a situation where despair is the dominant emotion. A contrast is developed between unrequited love and the glory of the poet's art. The red roses in snow are one symbol of this contrast.
Triumph is accomplished, the poet indicates, through his own art. His song will be so moving that it will transform the beloved and move her to tears in spite of her coldness. Roses amid the snow can more meaningfully be interpreted as representing salvation rather than the cruelty of love. Through poetry, not only will the speaker see the beloved's eyes bathed in tears but a transformation of nature as well: roses that improbably bloom amid snow will reveal ivory and will immortalize one who gazes upon them by changing him to marble.
The essential meaning is that a higher love awaits the speaker rather than the literal love a woman might grant him. In the sestet, he states that he does not regret this denial of love but glories in it. The implication, I believe, is that his reward will come after death, in the fame that he will have achieved:
e tutto quel per che nel viver breve
non rincresco a me stesso, anzi mi glorio
d'esser servato a la stagion piú tarda.
These lines have been translated into English in numerous and interestingly inconsistent ways. A more literal rendering than the attempts by translators to recreate Petrarch's poetic language might be something like:
And all this, in the brief life, I myself do not regret, but rather, I glory in being served in a later age.
This "later age (or season)"—la stagion piú tarda—apparently means the next life. The transforming roses envisioned in snow are a symbol of this latter-day glory the poet will achieve, despite the denial of his love in this life.