Undoubtedly what you did of committing fornication is a grave and major sin; rather it is one of the worst of sins after shirk and murder.
It is sufficient in affirming the gravity of fornication to note that Allah, may He be glorified and exalted, threatens the fornicators with a doubled punishment and eternity in the Fire of Hell, humiliated and disgraced, because of the gravity of their crime and the abhorrence of their actions, as He says (interpretation of the meaning):
“And those who invoke not any other ilaah (god) along with Allaah, nor kill such person as Allaah has forbidden, except for just cause, nor commit illegal sexual intercourse __ and whoever does this shall receive the punishment.
69. The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace;
70. Except those who repent and believe (in Islamic Monotheism), and do righteous deeds”
So zina (fornication, adultery) is mentioned alongside shirk and murder, and the recompense for that is eternity in Hell with a doubled torment and disgrace, if the individual who deserves that has not escaped it by means of repenting, believing and doing righteous deeds. Allah, may He be exalted, says (interpretation of the meaning)
“And come not near to unlawful sex. Verily, it is a Faahishah (i.e. anything that transgresses its limits: a great sin), and an evil way”
So Allah tells us that it is evil in and of itself, and it is an action that is so abhorrent that it is acknowledged as such by all people of reason. End quote from al-Jawaab al-Kaafi, p. 105
Zina (fornication/adultery) leads to eternity in Hell; what is meant by eternity here is a lengthy stay, the length of which no one knows except Allah.
Shaykh Ibn Baaz (may Allah have mercy on him) said: “Eternity” is of two types:
Eternity for which there is no end; this is the eternity of the kuffaar, in which they will never come out of Hell, as Allah says (interpretation of the meaning):
“Thus Allaah will show them their deeds as regrets for them. And they will never get out of the Fire”
The second type is temporary and will have an end. This is the eternity of some sinners, such as the one who kills himself, the one who commits zina, the one who deals with riba, and so on.
End quote from Majmoo‘ Fataawa Ibn Baaz 30/303
What you have to do is hasten to repent to Allah and to give up what you are doing, and you have to do a lot of righteous deeds, as Allah says (interpretation of the meaning):
“Except those who repent and believe (in Islamic Monotheism), and do righteous deeds; for those, Allaah will change their sins into good deeds, and Allaah is Oft‑Forgiving, Most Merciful.
71. And whosoever repents and does righteous good deeds; then verily, he repents towards Allaah with true repentance”
There is nothing in the shar‘i texts to indicate that the one who commits fornication with a virgin girl will not smell the fragrance of Paradise, especially if he repents and mends his ways.
And Allah knows best.