I second that: don't believe anything you hear. (Except this!)...(And all previous advice!):
I actually think that your wife is correct in challenging the "bipolar" diagnosis. I do think that she is depressed. I'll hazard a guess that she may have been depressed for years. But the person you describe is someone cyclically angry, with touches of paranoia and self-hate--and very loving and supportive between times. She doesn't seem to have episodes of mania.
I'm no expert, but a very, very similar cycle runs in my own family. My brother and I, as kids, took to spiking my mother's hot tea with a tranquilizer...which was very bad, but we lived in fear of her rages. Between times, she was a regular Earth Mother. There seemed to be no connection between Good Momma and Bad Momma. They seemed to hold no communion with each other. To this day, my mother has no clue about the kind of destruction she was capable of. She's mellowed; she's a good lady--always was. Back in the day, her rages had seemed to be in some way linked to her periods--like some kind of insanely magnified PMS.
The thing that sets your wife apart is that she does have a clue about what she's doing. She may feel helpless to stop that train. It's about as overwhelming for the person experiencing the anger and depression as for any bystanders mowed down. But if things get bad enough for her, she may choose to do the work she must do in order to be the person she was meant to be.
I say this, because, early in my marriage, I started reenacting the same cycle of sweetness and rage that had made my own childhood so unstable. It was only when my husband actually left me that I panicked and sought help. Counseling didn't help me. It's hard to find a counselor who is both sensitive and very smart. Anti-depressants were my life-saver.
So, I think that this is the time for you to focus completely on yourself. Your wife HAS to find her own way. It seems counterintuitive to lure her back by ignoring her, but it sounds like you've tried the loving approach on enough occasions to convince you that it ain't the way to go.
Give her space. Give yourself space. She knows you love her. Now's the time for you to do things that will make YOU happy. I think that you must stop pursuing her. Allow her to contact you. Be pleasant and kind, but brief. Go hiking, work out, write a novel, do anything that pleases you and takes your mind off your troubles.
When your wife calms down, she's going to start thinking...
