I'm browsing through my (soon to be ex) girlfriend's discussion board
Why are you looking through her discussion board?
I want to break up with her now that I've seen what she wrote about me
Normally you should trust your partner in a relationship, and not spy on her personal correspondence.  However, even though you probably shouldn't have seen it, your girlfriend should not be talking about you like this.  It is probably right for you to end the relationship.
She's a bitch and I'm making her pay big time!
I get that your angry, but allowing you emotions to dictate what you are doing is wrong. Just tell her what she did and end it.