Failures simply lead us closer to success
"There are no dead ends. There is always a way out. What you learn in one failure, you utilize in your next success."
-Henry Ford
These wise words are good ones to remember if you are in an unhappy relationship which may be coming to an end.
Don't think of it as a failure, or of that person as being the only one for you. Instead, think of this ending as a release into a new life, new chances, a great chance to be alone and please yourself for a change, or a clearing of the way for a better match elsewhere when you meet someone new.
Think of an ending as something that adds to all that you have learned, which you can take into your next chapter, whatever that may be.