I'm mad at my girlfriend for not standing up for herself when a guy offered money to her for a blowjob
So you're mad that someone tried to bribe her into performing a sexual act or because she didn't refuse?
I'm actually mad that she didn't refuse.
If your girlfriend didn't refuse performing a sexual act for money then that means she engaged in prostitution--something that is both morally wrong and illegal in many jurisdictions. You are right to be upset at your girlfriend for what she did as many would consider her actions to be inexcusable. You should have a frank discussion with your girlfriend and explain how hurt you are by her actions and see if she is willing to make changes to save the relationship or would rather break up.
Fuck, I think I need to break things off with her.
That's fair considering what she did. But at least talk to her first before making any final decisions.