Can the Confiteor be a replacement for the sacrament of reconciliation to remit mortal sin?
Can the Confiteor prayer as a short-term replacement for the sacrament of reconciliation during mass so you can receive communion even if you are not in a state of grace at that time?
The short answer to your question is no, the Confiteor does not take the place of the Sacrament of Reconciliation for the remission of mortal sin. The Catechism of the Catholic Church makes this clear in paragraph 1385:
To respond to this invitation we must prepare ourselves for so great and so holy a moment. St. Paul urges us to examine our conscience: "Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of profaning the body and blood of the Lord. Let a man examine himself, and so eat of the bread and drink of the cup. For any one who eats and drinks without discerning the body eats and drinks judgment upon himself." Anyone conscious of a grave sin must receive the sacrament of Reconciliation before coming to communion (emphasis added).
If you are conscious of grave sin prior to receiving Holy Communion, you should refrain from receiving until you’ve had recourse to the Sacrament of Reconciliation.