Why is everlasting punishment so difficult for us Christians to accept?
As mortal beings, everlasting is not a perspective we too easily grasp. Only through our spiritual self can we gain any since of the "eternal" consequence of sin. For most, even our view of punishment is temporary then "life" goes on. As "Christians" we are engrained with the acts of forgiveness and redemption not everlasting punishment nor the wrath of God.
In what ways might eternal punishment be considered just punishment?
As God offers salvation and life eternal with him, then to reject that is in an equal and logical sense death eternal.