I think I'm going to ditch my girlfriend on her birthday to study
Why would you ditch her on her birthday?
The stupid slut made me hate her and I'm sick of spending the night with her.
While you are great at keeping diligence with your studies, your girlfriend should be a priority as well. A birthday is only once a year and you are an unthoughtful person for just trying to brush that aside. If you are really sick of her just break up.
What, don't lecture me about my own life.
It is good to study but very wrong to abandon your partner on their birthday. If you really need to study you should tell them they may be very understanding. But if you lie they might find out and feel really hurt.