Scrupulosity is characterized by an excessive focus on one's sins, even when they have been forgiven in the Sacrament of Penance. Sometimes the penitent doesn't remember if they confessed the sin accurately or entirely. Other times a person may be tormented by thinking they are sinning,when they aren't.
Priests who hear confessions can tell when someone suffers from scruples, and they can help the person put their mind at ease. This is why it's important that confessors be kind and gentle, as St. Alphonsus Liguori always strove to be.
This saint founded the Redemptorist order in the year 1732. It now has more than 4,000 priests and about 5,000 religious brothers.
Here is some excellent advice on combating scrupulosity, written by a Redemptorist priest.
Flickr photo by cliff1066