Shakespeare Poem?

"That Time of Year Thou Mayst In Me Behold" by William Shakespeare. I don't really understand this poem, i just don't get Shakespeare. So what are some literary devices (allusion, imagery, symbol, ect.) in the poem and specific examples of them being used in the poem. What is the meaning of he poem? What is the poet trying to express? What do you understand about the poem? Please just try to help me with these questions because its really confusing for me. Any ideas or guesses are welcome. Thank you so much!