We've been fighting like crazy and have threatened to walk out a number of times. Is this it?

It's not the end until someone actually walks out...and stops threatening to do so.

I had a friend tell me something about when he knew it was over with his then-wife. He said, "She stopped arguing with me. She just didn't care anymore." She mentally left the relationship before she left it physically. In fact, she ended up having an affair.

It's not over until you say it's over or she says it's over. But if you're worrying about it being over, you might cause it to be over. You know?