Lust is not only a mortal sin when acted upon. Harboring lustful thoughts is also a mortal sin.
Matthew 5:28- “But I say to you, that whosoever shall look on a woman to lust after her, hath already committed adultery with her in his heart.”
Here are a few more helpful quotes:
“More souls go to hell because of sins of the flesh than for any other reason.” - Our Lady of Fatima
"In the realm of evil thoughts none induces to sin as much as do thoughts that concern the pleasure of the flesh.” - St. Thomas Aquinas
“In temptations against chastity, the spiritual masters advise us, not so much to contend with the bad thought, as to turn the mind to some spiritual, or, at least, indifferent object. It is useful to combat other bad thoughts face to face, but not thoughts of impurity.” - St. Alphonse Liguori
“Chastity is the lily of virtues, and makes men almost equal to Angels. Everything is beautiful in accordance with its purity. Now the purity of man is chastity, which is called honesty, and the observance of it, honor and also integrity; and its contrary is called corruption; in short, it has this peculiar excellence above the other virtues, that it preserves both soul and body fair and unspotted.” - St. Francis de Sales