What are the literary devices in "Sonnet 91"?
As a way of beginning to answer this question, you might want to focus on the final rhyming couplet that concludes this excellent poem. Let us remind ourselves of what it says:
Wretched in this alone, that thou mayst take
All this away, and me most wretched make.
Having established how the love that his beloved has for him makes him the envy of men, no matter how rich they are or how noble they are in birth, this is a rather surprising closing for a number of reasons. Firstly, note that this couplet contains an oxymoron. We do not associate the state of being in love with wretchedness, yet this sonnet points towards the way in which love can cause happiness, but also pain and conccern. Normally, sonnets attempted to glorify and honour love, but Shakespeare deliberately goes against this tendency to create a shocking finale that points towards a deeper truth about the state of being in love and the concern and worry that it can cause.
You might like to think to about how the ending of this sonnet is a very shockingly honest expression of the feelings of the speaker, leaving him very vulnerable and open to pain. Thsi negative couplet stands in stark contrast to the rest of the poem, and is very different to the normal hyperbolic focus on love of other sonnets. Yet it is the turn in these two lines that make the poem so effective, as Shakespeare at once extols but also points out the perilous position of being in love. Central to understanding this poem is therefore appreciating the oxymoron at the end of the sonnet and how it contributes to the meaning of the poem as a whole. Hopefully you can use this as a basis to go and discover other literary devices. Good luck!