If I remember a sin after Father gave me absolution, I say that I want to include (insert particular sin) in my confession as well. He understands.
However, if a person deliberately omits a mortal sin, they make a bad confession (which is a sacrilege), none of their sins are forgiven and they cannot go to Holy Communion. They must make a general confession from the time of their last good confession to the present general confession.
When I was younger, there were times that I made a bad confession, being afraid to mention a mortal sin in confession because of what the priest would think of me. No suffering on earth can compare with the knowledge that one has made a bad confession. I had no peace of soul, was anxious, terrified… The only thing that helped me was a general confession, and then I regained peace in my soul.
I can honestly say that God in His Infinite Mercy has been and is good to me. Otherwise, I wouldn’t be posting this today. I’d be either in my grave or spending $$$ on psychiatrists.
NEVER be afraid to mention anything in confession. The Enemy will try to sow distrust towards your confessor. Even if you think it’s stupid (Oh, what will Father think of me?), mention it anyway.
Most of all, ask the Holy Spirit to help you examine your conscience and make a good confession.
If through the grace and mercy of God I get to Heaven, I want to thank Him for everything - salvation, my Catholic Faith, the Sacraments and especially the innumerable times He has forgiven me my sins and given Himself to me in Holy Communion.