Get a tox-screen to lend your excuse credibility and then you'll have to lie and tell her you were out of your mind. Sticking to the truth will only work if your girlfriend is enlightened to the point of recognising that conventional attractiveness is much less prevalent and important than society would like you to believe. But the want for ones partner to consider one beautiful probably runs deeper than societal indoctrination so even then it'll be an uphill battle.

What I'm trying to say is you either have to lie to her and everyone that was there that evening and continue that relationship based on lies. Or you stick to the truth hope for an incredible amount of forgiveness and make sure to never let that information out in your next relationship.