Thank you all for your observations and comments, i take them to heart, and appreciate them. It's a relief to hear from people in the know...
Without a doubt any number of incredibly reasonable, loving people would have left this situation a long time ago and not dealt with the outbursts or well any of the crap any of us bring to this relationship.
One thing that has kept me going through all this is that her emotional explosions have been followed by very reasonable discussions where we find that we're not that far from each other. Frankly her emotional side is refreshing to me as I come from a very stoic and reserved family and community. It seems like... a thing that could be worked through. I stick through my husband's depression, is this anger really that much worse than the neglect that is a side effect of depression?
In this conflict though she and I still land on seemingly opposing view points. And at the risk of making the conflict too much, many things said in the heat of the moment indicate big big big differences in how we operate in the world and relationships. So... If what she said in anger is true in calm, well it gives me cause to worry.
Why would I stay with her? Because of course this isn't the sum total of our relationship. There are times when it's fun and loving. There's a stretch here that's been particularly stressful, and my strength has waned. We are preparing for a commitment ceremony for the three of us. It would be this thing which is enhancing the stress. It's like any wedding after all, money, emotions, decision making, family and expectations all rolled into one big hurry up and get it planned. Being who I am, I must follow through the ceremony. Frankly, I'm really looking forward to it for so very many reasons. Relationship, party, location, ceremony, all of it. Hah! Afterall I do like officiating for a reason!
However, everyone's comments here have helped me to focus my... self. It's pretty safe to say that I'm just now, at the ripe old age of 35, learning what my personal boundaries are. It was not something I was strong with before. I know who I am, but I don't have a good "hey stop that I don't like it" trigger. I also think of Dan Savage and "the price of admission". I think it comes into play here.
The neat thing I've learned about being at the boundary is that being so emotionally drained makes it simple not to cave to the "I must make amends" urge. We'll see where this all ends up when I have opportunity to assert and address those boundaries.
Oh I am not the other koi lady, but I do remember being struck by the similarities in our situations.