Caught girlfriend making out with a guy on my Birthday. What should I do?

That's so sad. She should stop drinking so much if she's going to become so flirty and cause problems. I don't know what I would do - although she was drunk and it wasn't what she would usually do, I wouldn't be able to stop picturing them together. I think you should talk to her about it, sleep on it and decide yourself if you love her enough to try again.