When you submit the Poem and get the "You have submitted" page, that's it...your Poem is on the page. There is at this point no venue for correcting your Poem. Going back and changing something and clicking Submit will just add a second copy of the Poem. Likewise, if you go to the guestbook and see your Poem is not there, reload the guestbook with your browser and it should appear, don't go back and click Submit again.
You may use html coding to format your Poem.