Oh sweetie, I'm sorry you've been affected so deeply by this. I have a 15 year old daughter, and I can tell you that for some kids, it's a rough time emotionally, with LOTS of ups and downs.
I'm glad therapy helped, can I ask WHY you would read the poem again? You clearly are sensitive to those kinds of things, so you must make an effort to try to avoid things that are dark, or that give you a disturbing mental image.
Try to put it out of your mind, remind yourself of the things your therapist taught you, and DEFINITELY no more reading of the poem! Look at it like PTSD, like soldiers have after war? It's a very similar reaction, the poem triggered those thoughts and emotions in you.
If you can't shake it off, then definitely get back into therapy. And, keep talking to a trusted adult about how you're feeling. Best to you, keep in touch okay?
We also have a teen depression forum that you may find beneficial:
http://www.medhelp.org/forums/Teen-Depression/show/185