I think that you can look back and ammend what you did wrong in the future. You don't have to look back necceseraly just to regret what you could have done. You need to get over with it and look towards the future because going back to old memories won't get you out of the whole that you are in.
But its not just after a break up we tend to look back and dwell on what could have been but nearly most things. I suppose its the fact that well all do it and don't really stop our selves, well some might but not my self. Its just that its human to get upset and think of what could have been but is it really necessary? so what do you think?