When you see the love between you slowly drifting away. After putting all your effort and your trust in a relationship and see it walking away out of the door, slowly, everyday. It is when the nicest and purest of all intentions was perceived as controlling and choking. It is when you try to explain your side and the other person sees it as just another debate you want to win.
You know it is the end when after having done everything you think will save the relationship and still failed. Sad and bitter when there is nothing left to do that will revive the dying emotions. When both of you had done all the best you could but still not enough. When you agreed that the best thing left to do, for each one to be happy and free, is to part ways.