What are Besetting Sins

In the life of every individual, there is a besetting sin that persistently stands between him and God.
It differs from person to person. What is a besetting sin to one may not trouble another at all.

12 Therefore we also, since we are surrounded by so great a cloud of witnesses, let us lay aside every weight, and the sin which so easily ensnares us, and let us run with endurance the race that is set before us.
— Hebrews 12:1