Poems about insanity/madness?

I'm looking for poems by published authors that reference insanity, sanity and madness. Basically a text that subtly suggests 'if I'm not sane, I must be insane'. It is for a high-school English ignment! I have (so far) "Much Madness Is Divinest Sense" by Emily inson, any other suggestions along those lines? Cheers!