It is possible, when stress is chronic, to suffer the way you describe, loss of capacity, from a biological standpoint. Stress hormones can change your profile. However this is completely reversible.
*Eat more protein, and cholesterol (and good fats).
*Listen to binaural beats if you're not epileptic.
*Meditate daily, something like transcendental meditation, where you repeat a nonsense word over and over for a lengthening period of time. This will help with any stress you're having right now.
It may take a while, but things will improve.