austinrex08,
Your story bears a great deal of similarity to mine. I posted it in another forum (Bipolar Spouses), but to recap...
I spent twelve wonderful years with a man I wanted to spend the rest of my life with. After twelve years he asked me to marry him (on bended knee at a family dinner in front of all my relatives). I said, "Yes." One month later the mania started and lasted until he left me six months later to go home to his family. (He is undiagnosed but I did A LOT of reasearch trying to figure out what was happening. Someone on a marriage.com website suggested to me that he was bi-polar and recognized my husband's behaviors very well. I researched bi-polar and discovered that it fit exactly.
He came back four months after that (with a lot of encouragement from me) and then suffered severe clinical depression for almost two years. I still didn't truly understand the illness so it was a very difficult time for both of us. In hindsight, I regret I was very judgemental.
Then one day I noticed he was his former wonderful self 
. I was so happy. We got married. Five months later the mania started again. It took me a few months to realize it was happening again, and so, of course, it caused quite a bit of conflict in our relationship.
Interestingly enough, both manic periods started (or I identified them) in August, three years apart. I also realized that all his significant relationships (three of them) ended at about the twelve year mark. Upon further reflection I can honestly say that there were signs of mania throughout our first twelve years. We were incredibly in love, however, and I just thought he was an extremely upbeat, high energy kind of guy. When he would stay up for three days at a stretch making something I thought it was wonderful he had that much creative passion. There were times of incredible rage, but they were forgiven. There were also times of severe depression, but I don't remember them being long periods.
We have been married for nine months now and he has been manic (that I can tell) for the last four. I still didn't understand the illness and its effects, so it has been four months of argument and bad feelings. When he is manic he will just get in his car and drive off and be gone for 1-2 days with no care to explain what he was doing. He also spends a horrific amount of money and has gotten himself in severe financial trouble in just four months. Fortunately, our finances are still separate and I pay all the bills out of my income.
I finally ordered a book two weeks ago called "Loving Someone with Bipolar." It is a wonderful book and I highly recommend it.
This time around I am trying to do things differently so maybe he doesn't run off for a long period of time. I am also taking care of myself and have started to put together divorce paperwork - just in case. After almost sixteen years of loving him completely I am finding out that I forgot to also love (and honor) myself.
My advice to you...continue to love her and want the best for her. To the best of your ability, use this time to find out more about who you are and what you want out of life. Heal yourself. We are all co-dependent to some degree and if (when) she comes back she will need you to be the strongest, healthiest person you can be. She will most likely come back at some point and if you aren't healthy enough to deal with her and stay healthy you will find yourself in the same situation, repeating the same mistakes. You have choices here and you need to find clarifty about what you want YOUR choices to be. If, ultimatley, the relationship doesn't work you will be healthy enough to find a new, stronger relationship.
I am including the lyrics to a song that got me through my first (very ugly) divorce...
Patty Loveless
Hurt Me Bad (In a Real Good Way)
Album: Up Against My Heart
Would of never found you
If he had wanted to stay
Oh he hurt me bad
In a real good way
Well he opened my eyes
To a world beyond
That impossible dream
I was livin' on
And I thank my lucky stars each night
When you're in my arms
And your holding me tight
I realize what it means
To have your true love
I would of never found you
If he had wanted to stay
Oh he hurt me bad
In a real good way
And I thank my lucky stars each night
When you're in my arms
And your holding me tight
I realize what it means
To have your true love
When I look back on my life before
How my heart got shattered
With a slammin' door
I see how it all had to be
For the two of us
Oh the river of tears
That flow from my eyes
Was only moving me on
To this paradise
I would of never found you
If he had wanted to stay
Oh he hurt me bad
In a real good way
Oh he hurt me bad
In a real good way
