Charles H. Spurgeon on Psalms 119:81

My soul fainteth. Fainting is proper to the body, but here it is ascribed to the soul; as also in many other places. The Apostle saith, "Lest ye be wearied, and faint in your minds" (Hebrews 7:3); where two words are used, weariness and fainting, both taken from the body. Weariness is a lesser, fainting is a higher degree of deficiency: in weariness, the body requireth some rest or refreshment, when the active power is weakened, and the vital spirits and principles of motion are dulled; but, in fainting, the vital power is contracted, and retires, and leaveth the outward parts lifeless and senseless. When a man is wearied, his strength is abated; when he fainteth, he is quite spent. These things, by a metaphor, are applied to the soul, or mind. A man is weary, when the fortitude of his mind, his moral or spiritual strength, is broken, or begins to abate, when his soul sits uneasy under sufferings; but when he sinks under the burden of grievous, tedious, or long affliction, then he is said to faint, when all the reasons and grounds of his comfort are quite spent, and he can hold out no longer. Thomas Manton.