The only sins we are obligated to take to Christ in Confession are mortal sins. Three conditions must be met (and often are not or we are not sure):
- The matter is grave
- I did it with full knowledge (that this was a grave matter, a serious sin)
- I did it with deliberate consent (no pressures of any sort, no long-standing habit, etc.)
For these sins we must have contrition. This means to (a) be sorry for having committed them and (b) resolve with God’s grace never to commit them again. Without these resolutions, the confession is probably invalid.
The reason for being sorry could be
perfect (I am sorry because I offended God, who is all good) or
imperfect (I am afraid of burning in hell). Either way works. God knows you want to be with Him either way.
Then of course there’s reparation. How will you work to avoid the near occasion of sin? Will you do some restitution? What penance will you do (besides what may be assigned to you)? There is no safer life than the penitent life. Out of the Confessional, if forgiven, we are not sinless, but sinners on a path of perfection. There is much road to be traveled.
Now comes the sticky issue - what about sins I am unable to number, or that I may have forgotten? Begin by mentioning whether or not you did a good examination of conscience. Some do one every night. Some keep a journal of faults. Some write down their sins before going to confession, then destroy the paper. If none of this was done and you really can’t recall any old sins you never mention, rest assured that God has forgiven them. God’s forgiveness is all or nothing.
What makes a confession invalid? Willfully lying to the confessor. Willfully withholding a sin because one is ashamed. Saying one is sorry for a sin when one is not is a lie. Better tell the confessor: I did X and I do not feel sorry…help me, Father. This is honesty. God loves a honest heart. He may give you the gift of compunction (feeling sorry for the sin). Ask for that gift. Also sometimes we think something is a sin, but it’s not. Let the confessor address the issue. Talk to your confessor, he is there for you, for the sake of your soul. Do not fear him.