Nick
You are in a very difficult situation and I commend you for trying to understand where your fiance is coming from. Is she on medication for her disorder? Under the care of a doctor or therapist? Maybe a couple of joint sessions with her therapist could help you understand her more and how to handle her illness to increase the success of your relationship if your fiance would allow that.

It is a shame that she is shutting you out when you can offer he so much support, especially if she is going into a depressive phase. Your idea to ask her what she needs from you is a good one. Let her know that you are there for her for any support she may need. Hopefully she trusts you enough to turn to you. I think it is great that you understand that this mood is only temporary and that things will return to normal after a time. That is the nature of this illness.

however it can put a strain on the best of relationships. i would recommend really educating yourself on bipolar disorder. and when your fiance isnt cycling, have her tell you as much as she can about her personal cycles, what you can expect and what you can do to help and things you should steer clear of.

if you ever need to talk, feel free to PM me.