Hi Broken Dad
First off recognise that whatever you do it will be wrong!!!
Just kidding, obviously. But there is an element of truth in it. Often we assume we know what the other person wants and try to do it but actually it can be the complete opposite of what they want - I see this frequently when I'm coaching couples. So, the best thing to do is to ask her what she wants from you right now in terms of support. (Better not to do this in the middle of a mood swing moment if you value your life!).
Simple questions and with the language that works for the two of you. Eg 'When you're in one of those bonkers moments (or however the two of you refer to them) how do you want me to be?' 'Would you rather I clear off out of the way or does that make it feel worse?" Do this is a way that doesn't make her mood swings her fault / wrong or blame them as none of that will help.
Listen when she's ranting about something and try and look for what is the request behind the complaint. Eg she says 'you are a selfish pig, you leave rubbish everywhere and expect me to pick it up' actually means 'please can you be more tidy because I'm finding the mess overwhelming at the moment'. There will always be a request it might just be a bit hidden in rant.
Try not to react and go into combat mode yourself. Hard to do but no good will come of it. When an argument has happened try and build bridges or hold out an olive branch asap, in the long run you will be both benefit from it.
Show her you care. You will know what works with this - some people like small gifts, some like to be complimented, some just want a cuddle etc. Think back to when you last really made her smile and do more of that.
Good luck Broken Dad - hang on in there.
Sue,
http://www.thecouplescoach.co.uk
"Helping relationships go from good, or even pretty crap, to great (or at least better!)"