Re: EX-g/f Bday.
If you guys broke up, what's the point of going? I wouldn't even acknowledge her birthday. I'd just blow it off. You're not going out anymore, why should you say anything to her?
You need to go out with your friends and spend time with them. Who cares what she's doing or what day it is? She's not your girlfriend anymore. Just forget about it and move on. If you go to the party, you're going to be miserable seeing her flirting with other guys. If you call her on her birthday, you're just going to start rehashing all of the old BS about why you broke up in the first place. What's the point?
The only way to handle a breakup correctly is to totally cut that person out of your life. No contact again, ever. Because that is the only way to force yourself to get over them. If sometime in many years, you happen to run into her, fine. But you need to start focusing on your future without her now. There's no point in prolonging that part of it. It's time to just move on and forget about this one.