If I have killed someone and feel sorry about it, I could repent directly to God, but it would be difficult for me to know for sure that he has heard and forgiven me. God knows our human nature only too well. The Resurrected Lord appeared to his disciples, breathed on them and said to them, "Receive the Holy Spirit. Whose sins you forgive are forgiven them, and whose sins you retain are retained." (John 20.29.) Now we repent to the Lord before a priest. When the priest absolves us of our sin(s), we know we are being forgiven. The Holy Eucharist is to union with God as the sacrament of penance is to being forgiven by God. After receiving the Holy Eucharist, you know that the Lord is healing your soul to become one with you.