Associate
Never said it was easy and ultimately you'll have to find out what works for you. Being the designated driver will help if you can.
On the girlfriend front you might be surprised - I've found that sometimes people have used me as an excuse to leave earlier than they would overwise get away with. "I'm going to have to head, monkeys driving and wants to leave". Drivers and non drinkers get much more scope to politely excuse themselves than drinkers do. Not going to work every time, but the chances are if you're bored she might be too. Equally she might only be going to Uncle Bob's 67th birthday because you are. You might find she's as unenthusiastic about a night out as you are if if you're honest about not wanting to go. Again it won't happen every time but it might help 'limit your exposure'.
Of course all this leaving early and ducking out can make you feel like a dull boy. Personally I made a concerted effort to make time for friends doing non drinking things so my non attendance at night's out didn't affect our friendship. Sometimes I'll join them for dinner but leave shortly after. I'd also try and do nice things with your girlfriend on nights should be at Uncle Bobs 67th. Made me feel less like I was missing out.
My final piece of advice is don't make a big fuss about not drinking. When someone asks you what you'd like to drink just say 'Coke, please'. I used to think people would expect an explanation for my choice of non alcoholic drink but usually they don't and even if they do, its up to me to decide if I want to give it. I usually just say I'm driving if asked, even if I'm not.
