How to gently tell your current bf/gf that it is over?

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You almost have a perfect relationship with your bf/gf. You get along so well and very mcuh comfortable with each other. But deep inside you are yearning for something else which the relationship is not fulfilling. You deeply examined yourself why. But still the yearning persist. It is never the fault of the other person, that you are so sure. How would you tell the other person that it is over, without having that person get terribly hurt?