I'm here to pickup my girlfriend and if she isn't here in the next 30 minutes then I am going to leave.
Why would you want to leave her behind?
I have better things to do. I'm tired of her.
While it makes sense to be annoyed that someone is wasting your time, she is your girlfriend and should be picked up no matter how long it takes. How would you feel if your girlfriend refused to pick you up because a meeting had gone 40 minutes late?
Forget it, I just left.
You made your choice, and I think that your girlfriend won't be very happy with it. Most people would think you left far too soon and should have at least tried to call her first.