Sir Thomas Wyatt (1503-1542) usually wrote his poems in sonnet form, specifically, in the Italian or Petrarchan form, but in the case of "Is It Possible," he chose a form, or variant of a form, called the rondeau (or redouble rondeau).
The epigraph for this poem helps guide us to an understanding: "The Lover Recounteth the Variable Fancy of His Fickle Mistress." We can expect the theme, then, to be a lover's lament about his mistress's inconstancy. The poem's refrain--"Is It Possible"--signals the lover's amazement that his mistress has apparently changed her view of their love on a dime, that is, suddenly, without explanation, without preliminary discussion. A lament over a mistress's changeable nature is a very common theme in 16thC. English poetry.
For example, in the first stanza, Wyatt laments the suddenness of the change, particularly considering the importance of the subject:
That so high debate. . . . Should end so soon, and was begun so late.
The poet can't quite believe that he's been hit by a whirlwind of rejection at a point where he believes he and his mistress have achieved a long-standing relationship.
The second and third stanzas shift from a lament about the mistress's cruelty to the poet's surprise that, just when he thinks the relationship is over, his mistress apparently relents--"and thence for the relent." The poet still cannot accept the fact of his mistress's changeable nature:
Within one heart so diverse mind,/To change or turn as weather and wind. . . .
In other words, his mistress is like a weather vane, turning whichever way the wind blows.
He uses another metaphor for changeability in the fourth stanza when he asks if it's possible
To spy it in an eye/That turns as oft as chance on a die,/The truth whereof can any try. . . .
Wyatt compares his mistress's view of their relationship to the toss of a die (as in, a pair of dice) in a table game, noting that no one can perceive, by looking at her eyes, what she thinks at the moment about the relationship. Comparing his mistress's eyes (or, rather, the meaning within her eyes) to dice is particularly appropriate because, in the view of people in the 16thC, the "eyes are mirror to the soul." If one cannot see into the soul, one cannot see the truth.
In the fifth stanza, Wyatt continues to explore the theme of his mistress's changeable views, and the poet expresses his continued amazement that it is possible to be brought low from a high point and yet "to light soft," an indication that the result of the fall is not necessarily fatal to his relationship.
The concluding stanza advises men to trust in the outcome of a mistress's variable nature--"Trust therefore first and after preve"--because, even though one might be crushed by his mistress's cruelty, "all is possible."