You've said you've always drank at parties. This may sound over-simplified, but try not drinking for once at this party just to see if you enjoy the experience more.
I've been around drinkers socially lately and my stock answer for why I'm not drinking is that I can't drink due to complications with medication I'm on (which is a white lie but I don't care right now). I'm still not ready yet to say the reason I'm not drinking is because I'm a recovering alcoholic.