How I interpret one of my life's most major catalyst...

[Horuseus]

There is no reason for me to experience that at the age of 8 in a stable household

There was a reason I included that piece of literature, and it isn't something you go through as a Child. I'll paste some excerpts to help make it clear for you as granted, it's a lengthy read:

We have already noticed the chief psychological characteristics of all normal mystical development. We have seen that its essence consists in the effort to establish a new equilibrium, to get, as it were, a firm foothold upon transcendent levels of reality; and that in its path towards this consummation the self experiences a series of oscillations between “states of pleasure” and “states of pain.” Put in another way, it is an orderly movement of the whole consciousness towards higher centres, in which each intense and progressive affirmation fatigues the immature transcendental powers, and is paid for by a negation; a swing-back of the whole consciousness, a stagnation of intellect, a reaction of the emotions, or an inhibition of the will.

who has described with much elaboration of detail her symptoms and sufferings during the oncoming and duration of the Night—or, as she calls its intensest period the Mystic Death—traces its beginning in short recurrent states of privation, or dullness of feeling, such as ascetic writers call “aridity”: in which the self loses all interest in and affection for those divine realities which had previously filled its life.

This is a complete emotional lassitude: the disappearance of all the old ardours, now replaced by a callousness, a boredom, which the self detests but cannot overcome. It is the dismal condition of spiritual ennui which ascetic writers know so well under the name of “aridity,” and which psychologists look upon as the result of emotional fatigue

In other words, this phase you're going through is a necessary transition. Yes, this may have been something you felt all your life, though you're at that precipice where you can consciously move forward and address said imbalance.

except for two possibilities: Biological defect and/or metaphysical phenomenon. I'll let the seeker take their pick.

The fact that this scenario 'started' at the age of 8 lends itself to the idea of preprogrammed catalyst, or in other words, a lesson/challenge you consciously decided to set yourself, so neither.

Even if it were a biological defect, the probability is known prior to incarnation and taken into account. I'm not sure of the practical implications of this either, unless you're insinuating a physical defect within the mind inhibiting the feeling of emotion?