Question: Is it necessary for valid confession to be sorry for every sin of which one is aware of?
It seems to me that it is necessary to be sorry for every mortal sin of which one is aware, but that it is not necessary to be sorry for every venial sin of which one is aware and that in that case only sins for which one is sorry will be forgiven.
However, from the text of Act of Contrition which we say in the confession it seems that we need to be sorry for every sin:
O my God, I am heartily sorry for having offended Thee, and I detest all my sins because of thy just punishments, but most of all because they offend Thee, my God, who art all good and deserving of all my love. I firmly resolve with the help of Thy grace to sin no more and to avoid the near occasion of sin. Amen.