All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
There is no doubt that what you did is considered a sin and betrayal. If the situation is as you have described and you did what you did with your clothes on, meaning that there was no penetration of the penis into the vagina, then the Hadd for Zina (adultery/fornication) should not be applied, even if the case was referred to the concerned authorities (the Muslim ruler). What you are required to do is to turn to Allaah, the Exalted, in sincere repentance, seek forgiveness, and resolve not to re-commit this sin in the future. However, if there was penetration of the penis into the vagina—this constitutes Zina. The Hadd for Zina should be applied if the case was referred to the concerned authorities (the Muslim ruler) and there were four witnesses to the incident or the perpetrator swore an oath by Allaah, the Exalted—four times—that he committed Zina.
It is better for the sinner who has been afflicted with committing this sin to conceal it and to not expose himself. It is enough for him to turn to Allaah, the Exalted, in sincere repentance. Allaah, the Exalted, says (what means):
• {Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allaah. Indeed, Allaah forgives all sins. Indeed, it is He Who is the Forgiving, the Merciful."} [Quran 39:53]
• {And those who do not invoke with Allaah another deity or kill the soul which Allaah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse. And whoever should do that will meet a penalty. Multiplied for him is the punishment on the Day of Resurrection, and he will abide therein humiliated—except for those who repent, believe and do righteous work. For them Allaah will replace their evil deeds with good. And ever is Allaah Forgiving and Merciful.} [Quran 25.68-70]
The Prophet, sallallaahu ‘alayhi wa sallam, said: "... Whoever is afflicted by committing any of these ugly sins, he should cover them up with the veil of Allaah." [A-Haakim, Al-Bayhaqi; As-Suyooti: Saheeh and Al-‘Iraqi: Hasan]
As for you words, "Please, do not tell me to seek forgiveness, repent, and resolve not to recommit this sin," this is indicative of your fear of Allaah, the Exalted, sincere repentance, and keenness on what is good. However, you should know that this is the sharia ruling on this issue and that doing this will help you to purify yourself from your sin before meeting your Lord, as long as your repentance is sincere, Allaah willing. You should also command your fiancée to turn to Allaah, the Exalted, in sincere repentance.
Note: The punishment for the fornicator, if the due conditions of the application of the Hadd are fulfilled, is to receive100 lashes—not 80, as you have mentioned.
Allaah Knows best.