Imagine if you had to force yourself to not live in the past or the future, and just live in an endless present?
Prisoners have to do it.
Thinking of the future stirs up intense yearning and hope that they find unbearable. They quickly learn that these emotions make them vulnerable to disappointment. They therefore consciously narrow their attention, focusing on extremely limited goals.
And, the past like the future, becomes too painful to bear, for memory, like hope, brings back the yearning for all that has been lost.
Thus, they are eventually reduced to living in an endless present.
How painful and difficult must that be? Because this rupture of continuity between present and past frequently persists even after they are released.