Isn't is OK to confess in your heart if you are sincerely sorry?
Isn't confession to a priest an option? If you're sincerely sorry for your sins and confess them in your own heart, aren't you already forgiven?
The power to forgive sins was one Christ gave to his apostles (Lk 10:16, 2 Cor 5:18-20). After he rose from the dead Christ said to the apostles, "'As the Father has sent me, so I send you.' And when he had said this, he breathed on then and said to them, 'Receive the Holy Spirit. Whose sins you forgive are forgiven them, and whose sins you retain are retained'" (Jn 20:22-23).
We can be truly sorry for our sins--that is essential for forgiveness--but we can't forgive our own sins. We can't absolve ourselves. That is a power reserved to God alone. Through Christ that power was conferred on his apostles and their successors, the bishops, and their helpers, the priests. Confession is not an option. It is the ordinary (normative) means through which sins are forgiven.