I spoke to someone today who reminded me of this phenomena when experiencing heaven or hell.
The reality of the permanence of eternity, you somehow realize the enormity of the tangible presence of eternity.
Whether one goes to heaven or hell it is the same, one wonderful the other horrible, but eternal and unchangeable.
When one enters eternity there is no chance to change if you have gone on the wrong path, the eternal crushing reality is inescapable.
However, the eternal reality of heaven is relief, peace, thankfulness and joy  unspeakable.