Supposedly, this is a sonnet written about Thomas Wyatt's failed suit for the hand of Anne Boleyn—who, of course, was to marry King Henry VIII and, later, was beheaded on his orders. The poet repeats the refrain "forget not yet," prevailing upon his beloved—who is now promised to another man—not to forget how true he has been in his attentions to her.
Wyatt depicts his pursuit of the beloved as an endeavor: it is a "travail" and it has made him "weary" to continue in this "suit" and "service." He also feels he has not been treated correctly in his pursuit—he has made "great assays" and has maintained a high level of patience even while his beloved has denied him even any closure—she has not said either yes or no, but has allowed him to continue trying to pursue her, while she herself has behaved rather scornfully towards the suitor. She has not understood, seemingly, that he truly loved her, has done so for a long time, and has never thought of anyone else—his mind has never gone "amiss."
In the final quatrain, the speaker describes his own love for his beloved as a "steadfast faith," as one might describe faith in God. He is saying that he has loved her purely and for a long time, never thinking about anyone else. Although he now knows that he can never have her, the speaker is prevailing upon his beloved never to forget how dedicated the speaker has been to her for all this time.