When a person is jealous, it usually means he or she is insecure about something.
If jealousy lurks inside the woman you're involved with, there's a right way to handle it and a very, very wrong way. Do the wrong thing and watch the crazy combust.
The thing about jealousy is that it doesn't happen if you feel 100% confident, but no one, not even Beyonce, is 100% confident all the time. What I'm trying to say is that jealousy is almost inevitable.
The best way to deal with jealousy is conversation and logic. I know this can be extremely hard when your girlfriend has pushed you to the limit of your sanity, but you have to remember that she isn't psycho, she's just feeling insecure.
Try to think about why this might be. It's OK to ask, but you have to do it the right way. Take her aside, give her a hug and just say, "What's going on with you? Is everything OK?" Tone is so important here. There cannot be an inch of frustration in your voice. Deep breath, OK?
She might defensively react by putting the blame back on you or on some girl you were talking to. She might cry. She might deny that anything is wrong. Alcohol usually affects reaction. Is she drunk? Are you drunk? Will having a drunken talk really be helpful right now? If you are both drunk, it's best to separate and talk about this in the morning. There is nothing wrong with that. It's the smart choice
As with any conversation in which your ultimate goal is to make someone see things your way, you have to stay calm and use logic. You have to use your calm voice, because it will force her to use hers as well. Reminding her that you love her is the key here. You do, right? If you do, you can fix this. Remember, jealousy is insecurity. What fixes insecurity? Instilling confidence and love in that person. Be compassionate. If you yell, it's only going to get worse and fuel the fire. Trust is the key here. If you have never given your girlfriend a reason not to trust you (i.e. never cheating if you are monogamous), then you can say that. It's all about tone! I cannot stress this enough. Stay calm.
(However, if you try this technique over three times and she keeps acting insane, you have to call a spade a spade and realize that this jealous freak is not ready to recover yet. It might not be worth the drama.)