Although I’ve been to confession and felt sorrow for my sins of the past, I still struggle with them and they haunt me. How will I know I’ve been forgiven?
When the priest says the words “I absolve you from your sins, in the name of the Father, and of the Son, and of the Holy Spirit. Amen” you can know that you have been absolved from (i.e., forgiven) your sins. Just as the sacrament is not dependent on the personal holiness of the priest, so it is not dependent upon your own personal holiness. All that is necessary from you is that you be repentant – not necessarily feel sorrow – and that you be honest about your sins.
There is a difference between the will and the feelings. One cannot help what one feels. One can only choose what one wills. If you feel unforgiven, even though you know on an intellectual level that your sins are forgiven, pray for the grace that your feelings will be ordered to what you know is objectively true. Recognize that being haunted by forgiven sins is, in reality, temptation to despair and will to reject such temptations. Offer up the pain that such temptations cause you to Jesus on the cross. Although Jesus himself never sinned, he knows what it feels like to be tempted (cf. Matt. 4:1-11, Heb. 4:15)
Understanding Scrupulosity** by Fr. Thomas M. Santa, C.Ss.R.