How can I narrow down my focus in a paper comparing love themes between Wyatt and Shakespeare's sonnets?
First, I'd like to say that you are absolutely on the right track in your writing process here. First, you have chosen your authors. Next you have chosen your theme, and now you are working on narrowing your focus. This is precisely the right way to do prewriting for a literature paper.
There are two ways you can narrow your focus. One would be to select a limited number of works to discuss, perhaps no more than one or two works by each poet. The other would be to look at general differences in their approaches to love. As many of Wyatt's sonnets were translations of Petrarch, it is somewhat tricky to attribute specific thematic concerns to Wyatt in them, but still, he chose a specific group of poems to translate, and they do have certain elements in common.
First, Wyatt is more consistently negative, or pessimistic than Shakespeare, focusing mainly on elements of love that are painful or difficult. In Shakespeare's case, even when a poem starts out describing the poet's discontent, such as, "When in disgrace with fortune and men's eyes," normally the poem will have an optimistic resolution.
Wyatt also seems more intensely and abstractly introspective than Shakespeare. Often, rather than any concrete descriptions of his mistress, we get a close analysis of his emotional state, such as in "Love and fortune and my mind, rememb'rer ..."
One possible focus you could chose is the way the poets talk about bad Fortune in relation to love, with Shakespeare seeing love as an antidote to the whims of fortune and Wyatt seeing love and fortune as acting together, with unrequited love being part of ill fortune.