How did you know?

I have been so confused lately about the state of my marriage. I don't want to go into details because I just want to know your stories. For those of you who have been married for at least a couple of years and then called it quits, how did you know it was time? What was it that pushed you over the edge? Any stories help, but I especially am curious about the ones where you made the decision on your own and your husband was still either happy or neutral with the relationship, or at least still wanted to work things out.