I literally feel as though I'm dying as I write this because of my gf.
That sounds troubling. How is she responsible for your feelings of dying?
The only thing that I wish for now is to take out my pain on that bitch
I know that you are angry with your gf, but wishing ill-will towards her is not right. Talk to her and deal with your pain instead of wishing it on her.
How else would I get even with her?
Do you really want to get even with her, or do you just want her to understand how she's making you feel? It would be a good idea to talk to her and let her know she's making you feel bad. She may not even know she did anything to upset you.