Before resorting to the assistance of the authority, determine for yourself, what the essence of your sin. Clearly specify your action, which now brings a feeling of guilt. Answer the following questions: "Who else besides yourself was hurt by your actions?", "How could be avoided?" "What should be done in the future to make the error happen again?". Write down your answers on paper, this will help avoid vague wording. Such analysis is important in order to understand the situation and to plan ways of its correction. Paper can burn as an act of "forgiveness itself."
Confess in the Church, repent, and receive from the priest the grace of remission of sins, if you were brought up in the Catholic faith. Pray, read the Scriptures and lives of saints. You must show a desire to avoid later in life repetition of sin.
Pray to the Lord, the Mother of God, read the akathist to the "Cover", if you were brought up in the Orthodox faith. To bring repentance needed in the Church through the priest. Church canons distinguish between fornication and adultery. For fornication excommunicated from the Communion to 7 years, and for adultery - to 12. Not looking for compassionate priests who will give penance for two weeks and is already pushing to Communion.
Refer to the Quran, go to mosque, if your religion Islam. Sin committed against their will is unforgivable. You need to repent and perform many good deeds, so that in the Day of judgement your good deeds could outweigh your sin.
Search for additional authorities, if you are an adherent of another faith. Seek help from a psychologist if that's more suitable. Perhaps you need to speak with someone from your family who can understand and support you not to despair. The Buddha is quoted: "Everyone is his own refuge, who else could be refuge?". But we need clarity of sight to continue his journey.