My girlfriend and I had a threesome when we were both drunk. Now she's really upset with me because she said I shouldn't have let it happen. I thought she wanted to. What can I do to help her feel okay about it?

That’s like getting pissed and deciding that parachuting is a great idea and going ahead without any preparation! Dangerous! As you’ve found out. Whenever you want to try something new, particularly something so potentially confronting as a threesome, it’s important that you proceed with caution and safety. The false bravado you get from alcohol is no comfort in the bright light of day when you’re feeling the disgust and self-loathing of what you blearily remember doing the night before. Now let’s be honest here, you claim you thought she wanted it, yet it’s clearly far enough out of her regular comfort zone that you would have realised that all was not normal, ie she was drunk and not her usual self. So her claim that you shouldn’t have let it happen is valid. I suspect you couldn't believe your luck and just went ahead without really checking in to see if she was ok with it or sober enough to make an informed decision. So you need to apologise to her first. Then the two of you need to have a open conversation about what your fantasies and turn-ons are and how you could gently, slowly and safely explore those new areas.