Heartfading

The perpetuals have always been rare, scarcer than nuggets of gold in a prospector's pan, more so now than during the age of the Aen_evë. Every race has its perpetuals, every species its individuals who live far past the normal span of their brethren, for reasons yet beyond the ken of man.
Perpetuals enjoy much more of life than the rest of us fleeting mortals, and can watch the world change beyond the breadth of the single lifespan that should have been apportioned to them. The life of a perpetual, however, is far from perfect, as they are uniquely vulnerable to a condition that the rest of us should be so lucky we never have to suffer.
To understand this condition, one must know that the mortal mind was not made to contain the entirety of a person's essence, the sum total of their experiences. The mortal shell can hold many memories, but not infinitely so.
For those who live normal lives, there is space enough in the mind so that memories, from childhood to old age, can be remembered well enough. For the perpetuals, such is not the case. The more that they see of their future, the more that they lose their past.
It is called heartfading, a needlessly poetic term for a thing so tragic, and yet one that bears thinking nonetheless. The core of a man is the sum of his experiences, and our earliest years form the foundation of who we know ourselves to be.
Many perpetuals, those that are known to science, fall despondent toward the end, as they struggle to remember the faces of their parents and their loved ones, as they fail to recall the summers of their youth, the lessons that they learned through blood, sweat, and tears.
Heartfading chips away at what makes perpetuals people, fraying the line that keeps them tethered to the world. In the end, many perpetuals simply lose the will to live, unable to feel any attachment to their past. Hardly any are motivated to end their lives by their own hands, but their bodies simply shut down as they increasingly refuse food and drink, eventually passing with a sigh.

Heartfading, De'Aldemenë Di'Karra (lit. Fading of the Heart) is a condition that transpires as a consequence of extreme old age. It occurs primarily in Perpetuals, individuals belonging to a species that live far beyond the normal lifespan of their kind. Though ill-understood by Dominion science, owing in part to their extreme rarity, it is believed that perpetuals suffer from heartfading because of the limitations of the mortal body, particularly the brain.
It is known that brain matter, like much of the body, degrades as a result of aging. This, compounded by the brain's finite capacity for information and memories, means that an individual who lives long enough will gradually lose old memories as new ones take their place without the aid of something external that can help them retain more information. It is believed that only perpetuals are affected by this condition because only perpetuals live long enough to experience it but, theoretically, any individual who lives beyond the span of their apportioned years should experience the same fate.