I wrote a paper about WCS's "The Jungle" in grad school for Al Gelpi. Of course I memorized the poem, and I still know it, though I think I had to re-learn it at some point, complete with line breaks. "It is not the still weight of the trees, the breathless interior of the wood, tangled with wrist-thick vine, the flies, reptiles, the forever fearful monkeys, screaming and running, in the branches..."
I remember I went to the rare book room and compared the original periodical publication of the poem with the final version.
When you forget a poem, a technique for remembering it is to not think about it too much, the word that you have forgotten, just see if you tongue provides it at the right moment.