If you've been in any kind of relationship with a woman, you've probably gone on or been taken on a rollercoaster ride at some point. Rollercoaster rides are those emotional "journeys" (to be nice) than she goes on before calming down and looking at things from a logical standpoint.
Now a lot of people say that men are creatures of logic, and women creatures of emotion. This is true to a certain extent. To say that all women NEVER use logic and have no use for it wouldn't just be sexist, it's also stupid. Some of the most intelligent movers and shakers in the world are women, and you don't get to that position by being an idiot. I'd argue that in personal situations, women get through it by focusing on emotion over logic. This doesn't mean that they don't care about logic in the situations, just that when they happen, emotion comes first and once they've worked their feelings out, solutions fall in line.
What does this mean? Don't try to fix it for her. Let her sort out whatever is going on in her head the way that works best for her. Trust me on this. You want to be there to encourage, reassure, or guide if needed, but the last thing you want to do is bust into her emotional place of coping with your logic guns blazing, trying to save the day on some Vietnam shit. What looks like a broken, emotional hazard zone to you is actually comfort to her.
In other words, when she looks like she's losing it (sometimes she might REALLY be losing it, but you should be able to tell the difference. Know your woman.) she's actually in a comfortable place. I read somewhere that some people actually use "depression" (I use quotes because I'm not talking depression in the clinical sense) as a coping mechanism, because it allows them to go through a range of emotions and FEEL something, at which point they can sort out their thoughts before leveling it out. It's not how your common man would deal with something, but that's why you're in a relationship with a woman, not a man.
What you want to do is let her know you're there if she needs you, and let her be. What you DON"T want to do is get as emotional as she is. That's a VERY bad idea. She needs you to add stabilization. You remaining calms allows her the comfort and freedom to spazz out and go through her range of emotions because she's in a "safe" place. If you're hyping the situation up by getting emotional too, not only do you make her uneasy, but you lose attractiveness because you're, well... acting like a woman.
This can be tough, because if you're like me, you hate seeing your lady upset--but honestly, sometimes you just have to let them cry it out. If I drop dead tomorrow she'll still have the same emotions to deal with for the rest of her life, so why should I make a habit of jumping in and being her emotional fixer?
Stand your ground if she's taking a bad day out on you, but outside of that, don't be afraid to just chalk it up as her needing some time to calm down and sort things out before coming back to the world. If you stop everything to "have a talk" about why she's feeling that way, you're setting yourself up for frustration. Just stay cool, let her know you have her back, and let her go on that ride by herself.
If you have a good woman, when she's back from that ride she'll be refreshed and thankful that you were there. Because if you did it right, you WERE there, even though you didn't go on that ride.