Dude - LTB & I have been to the brink more than once. Frighteningly close in fact, to the point where I was in tears huddled up in a puddle where nobody could see. Why & how we pulled back remains a mystery to me, so if you're looking for a "what could I have done differently?" kinda thing, I'm not the guy, and neither is she.
What I can tell you with absolute certainty is that shit happens, and more often than not it hurts. There is no other explanation sometimes, and the only "advice" I can offer with any amount of self-respect is this - you know who you are, and you know what kind of a person you are. Never let life's nutkicks make you forget that, and never allow the things that go south in life become the things that you allow yourself to be defined by.
I know you'd like some kind of "explanation", if for no other reason than to maybe gain some insight into how to keep the situation from repeating itself further on up the road. There may indeed be one that you're not getting, but then again, maybe not. Like I said, sometimes shit really does happen "just because".
Ok, one more piece of advice from the heart - you don't have to forget, but you defiitely need to forgive at some point. Hanging on to pain and bitterness is a recipie for long-term emotional suicide. It ain't gonna happen all at once, obviously, but it does need to happen. I do know that the only reason that Brenda & I have managed to stay together after pulling back from the brink those times is that we've made concerted efforts to let go of the pain we've caused each other over the years. And yes, letting go is an ongoing process. One might even say that it's a life's work. But there it is.
What I'm saying is that this unfortunate turn of events ain't gonna kill you unless you let it. But it ain't gonna make you stronger either unless you let it. I feel your pain, and wish that it wasn't happening. But since it is, all I can offer you is symapthy and the exhortation to keep your head to the sky, even if it means using a crowbar for support.