a big fight

My husband, I can hardly call him that now, and I have had a big fight.  I just got back from NYC and have had the worst trip ever after having our biggest fight ever.  It's not that anything was said that wasn't said before or that I was more angry than before but I think what made it so bad is that we have talked so much about those previous fights and thought we had come to some understanding only to find out we know nothing about how to make a relationship work and we are drowning.  he's not speaking to me right now nor do I want him to (well, yes, a small part of me, okay a big part of me wishes he'd come find me in the living room and pull himself away from ESPN) and he told me that it was "over" last night.  Over-how scary especially for a girl who has been living in fear all of her life that she'd find a great thing and then it would leave...ahh, manifesting my dreams, I guess...anyway, how do you know when it's time to throw in the towel or when it's just a hurdle, an opportunity to learn and build on?  How do you know when it is actually time to walk away?  I wish I knew