Some prefer to render the last clause, "In sorrow of heart the breath is oppressed."

By sorrow of the heart the spirit is broken - Every kind of sorrow worketh death, but that which is the offspring of true repentance. This alone is healthful to the soul. The indulgence of a disposition to sighing tends to destroy life. Every deep sigh throws off a portion of the vital energy.

A merry heart maketh a cheerful countenance,.... Or, a "joyful heart" (c); that is joyful in the God of its salvation; that rejoices in Christ Jesus; is filled with joy and peace through believing in him, in his person, blood, righteousness, and sacrifice; that has a comfortable view of his justification by his righteousness, of peace and pardon by his blood, of the atonement of his sins by his sacrifice; to whom he has said, "be of good cheer, thy sins are forgiven thee", Matthew 9:2; who has peace in him, though tribulation in the world: as such a man's heart must be made glad, this will make his countenance cheerful, or cause him to lift up his head with joy; as it is in natural things, so it is in spiritual ones;
but by sorrow of the heart the spirit is broken; a man is dejected, his spirits sink, and it is seen in his countenance: there is a great sympathy between the body and mind, the one is much affected by the other; when the heart is full of sorrow, the animal spirits are low, the nerves are loosened, the whole frame, of nature is enfeebled, and the body emaciated; this is often the case through outward troubles (d): physicians say (e) that grief weakens the strength, and destroys the spirits, more than labour does. "The sorrow of the world worketh death", 2-Corinthians 7:10; and sometimes, through spiritual troubles, a sense of sin and guilt of it, a legal sorrow, which produces a legal contrition of spirit; and such "a wounded spirit who can bear?" Proverbs 18:14. This is the effect of a mere work of the law upon the conscience; and stands opposed to the spiritual joy, and the effects of it, the Gospel brings.
(c) "cor gaudens", V. L. Baynus. (d) "Frangit fortia corda dolor", Tibullus, l. 3. Eleg. 2. v. 6. (e) Fernel. Method. Medendi, l. 7. c. 9. p. 54.

A gloomy, impatient, unthankful spirit, springing from pride and undue attachment to worldly objects, renders a man uneasy to himself and others.

maketh . . . countenance--or, "benefits the countenance."
spirit is broken--and so the countenance is sad.

13 A joyful heart maketh the countenance cheerful;
But in sorrow of the heart the spirit is broken.
The expression of the countenance, as well as the spiritual habitus of a man, is conditioned by the state of the heart. A joyful heart maketh the countenance טוב, which means friendly, but here happy-looking = cheerful (for טוב ro is the most general designation of that which makes an impression which is pleasant to the senses or to the mind); on the contrary, with sorrow of heart (עצּבת, constr. of עצּבת, Proverbs 10:10, as חטאת = חטּאת, from חטּאה) there is connected a stricken, broken, downcast heart; the spiritual functions of the man are paralyzed; self-confidence, without which energetic action is impossible, is shattered; he appears discouraged, whereby רוּח is thought of as the power of self-consciousness and of self-determination, but לב, as our "Gemt" [animus], as the oneness of thinking and willing, and thus as the seat of determination, which decides the intellectual-corporeal life-expression of the man, or without being able to be wholly restrained, communicates itself to them. The ב of וּבעצּבת is, as Proverbs 15:16., Proverbs 16:8; Proverbs 17:1, meant in the force of being together or along with, so that רוּח נכאה do not need to be taken separate from each other as subject and predicate: the sense of the noun-clause is in the ב, as e.g., also Proverbs 7:23 (it is about his life, i.e., it concerns his life). Elsewhere the crushed spirit, like the broken heart, is equivalent to the heart despairing in itself and prepared for grace. The heart with a more clouded mien may be well, for sorrow has in it a healing power (Ecclesiastes 7:3). But here the matter is the general psychological truth, that the corporeal and spiritual life of man has its regulator in the heart, and that the condition of the heart leaves its stamp on the appearance and on the activity of the man. The translation of the רוח נכאה by "oppressed breath" (Umbreit, Hitzig) is impossible; the breath cannot be spoken of as broken.