Sorry I am not all that into poetry myself. I hate to be the devils advocate, but what makes you think that a poem will win her back if she already left you? Also if she is already with someone else... then you are just setting yourself up for failure.
Good luck the_only_one, I am in a similar situation in which my girlfriend finished it because of stress. I'm trying to give her as much space as possible at the moment as she sorts things out whilst also reassuring her that I will be there for her should she need any additional support. I've made some bad mistakes this week however by getting insecure and not allowing her 'to think', so would just like to say try not to suffocate her (I'm sure you're not and you're doing a better job than I but just a bit of personal experience I hope might help a little )