Because the question has been edited, so will be my answer.
A Catholic should continuously make efforts to deepen his faith and the knowledge of the teachings of the Catholic church, to form his consciousness to be able to distinguish what is a sin and what is not, but finally, nobody is a judge in his own case.
I think this may be thought as the reason for a practice of Confession. Of course, the validity of sin reconciliation is not due to some rationally motivated reasons (at least the latter was my private opinion I believe to not disagree with the Church teaching), but due to Christ's command: "Whose soever sins ye remit, they are remitted unto them; and whose soever sins ye retain, they are retained." (John 20,23).
Because the question suggests that it's case of latae sententiae excommunication, it's not possible to tell whether those sins can be reconciled by any priest in any place (or maybe only a bishop), however without the person directing his or her steps to the confessional nothing can be stated.
As I stated in the previous version of this answer, lack of faith in sin remission is a very serious sin. Jesus took the penance for our sins for us not to doubt God wants to see everyone in Heaven, so belief that one has done anything that can not be forgiven (even if we are talking of very serious sins) is a disrespect to Christ and his Passion.
The teaching of the sins that are mortal, and those that are not is a reason for another discussion. Basically we believe that in case of any doubt, whether we are in mortal sin we should go to Confession before we can
receive the Communion (because we believe that the soul should be pure to show the respect for He who is present in the Communion).
Actually, I'm not a priest, but I believe not to have said anything that is in contrary to the Catholic faith.