I suppose, in an effort to remove bias from this question, the relationship can't still exist. I am referring to your most successful relationship that has ended. I beleive if you are currently in the relationship, it would be a biased opinion as you have not had time to reflect back on the relationship with an unbiased opinion.
The reason it is biased is because when you are in love with someone currently, and vice versa, you can't imagine anything that could of prevented both of you to not have come together as your current bond is so valuable. To think otherwise is to devalue your current bond you have right now and part of being in love is the notion so long as you walked passed each other, even if blind and deaf, and I the middle of a war zone, your hearts were meant to be together by fate but is not reality is it.