I'm going to go visit my girlfriend instead of going to a friend's birthday party
Why don't you want to go to your friend's party?
I don't know if I ever want to talk to him again.
It's okay to want to be with your girlfriend, but a friend's birthday is a special occasion, that should take priority for that day.  If you really don't want to talk to your friend again, you should let him know what is bothering you, instead of just abandoning him on what should be a happy day.
It's my birthday too and I want to celebrate it with my girlfriend. His birthday isn't as important as mine and I don't want to do anything else.
While I get where you are coming from, the least you could do is let your friend know, even if it might upset him. He's probably expecting you at the party.