If we could keep the promise of everlasting love there would be no unhappy marriages or unhappy human relationships of any kind. No one can keep that promise.
Happiness is a positive input of pleasure, and not merely a reduction of pain. If one gives a beggar on the streets of Calcutta a half of loaf of stale bread, and it is more that he has ever had, and it is more than his beggar friends have, he will be happy. Happiness is relative to our experience.