There is a beautiful phrase in Chinese “缘尽于此”. My attempt to translate it and express this feeling is “That predestined realtionship ends here” or " “the serendipity has run its course” or " “the serendipity has played itself out”.
Let me try to describe this feeling:
I met someone by serendipity. It was destiny that brought us together. And this force continued to guide us, so we had chance to experience so many things together, and maintain a good relationship for a time. Then the relationship ran its course. Now that binding force was gone, fate has decided that at this point we shall part, and never see each other again. The serendipity and luck in terms of that realtionship have played themselves out and there is nothing left now. It has ended.