Lower your expectations.
As weird as it sounds, the contrived professionalism of an office environment lends itself to fantasy. After maintaining a strictly Microsoft Office-based connection for the last 364 days, the office party provides an excuse you and your “work crush” to tear down that artificial barrier with reckless, carnal abandon. Plus, while at the office, you’re impervious to the watchful eye of your girlfriend, which might be another positive, particularly if you’re a jerk-off.
But, if you’re being honest and hyper self-aware, your connection is likely exaggerated and this chick’s probably as different on her own time as you are. There’s a lot that can go to shit if you mess with a coworker, so don’t take this holiday party (and all its potential revelry) lightly. Or do take it lightly. I don’t really care, TBH.