Is this wrong? I feel really bad....?

Personally I don't think it's wrong. I've always kept my work life and personal life seperate! I think you should say you are unable to make it and tell the truth that you can't make it because you don't have a lift. If one is offered to you ask if the driver will be drinking and if they are you have a perfect excuse to decline. Give your co worker a little git before the night out and just explain that you can't make it out for drinks but you just wanted to wish her well with a token present! All will be fine! Your boss won't ditch you over a little thing like this if he does it's unfair dismissal!