I plan on leaving the birthday dinner that my girlfriend is having since she was over 30 minutes late.
How did your girlfriend feel about you leaving?
She was really mad, but I am not concerned about it.
It is understandable that you are upset with her for not showing up on time. You should have a conversation with her so that in the future she will understand that you do not like having to wait for her.
Fuck that, who needs her dumb ass.
You have a right to be upset with your partner if they show up pretty late to a party. However, instead of leaving it may be better to just let them know how you feel. This is the only way that the situation will truly be fixed.