He committed a sin and his conscience is troubling him

What we think is that if a person repents from any sin,
Allaah accepts his repentance, because Allaah has mentioned the major sins
(interpretation of the meaning):  

“And those who invoke not any other ilaah (god) along with
Allaah,” – this is shirk – “ nor kill such
person as Allaah has forbidden, except for just cause,” – this is murder
– “nor commit illegal sexual intercourse” – this is immorality,
transgression against Allaah, against other people and against their honour.
Then Allaah says: 

–– 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; for those, Allaah will change their
sins into good deeds, and Allaah is Oft‑Forgiving, Most Merciful”

[al-Furqaan 25:68-70]

A person must repent sincerely, fulfilling all the five well
known conditions of repentance, which are regret, giving up the sin,
resolving never to go back to it in the future, sincerity, which is the
basic requirement, and repenting before the time for repentance is over. If
these five conditions are met then this is sincere repentance, by means of
which Allaah erases all sins that came before, even zina. 

With regard to the woman with whom you committed zina, if she
did that willingly, then she will be questioned about that herself. If she
was forced, then the one who forced her has to repent to Allaah from that.
If he is able to ask her forgiveness, then let him do so, otherwise let him
pray a great deal for forgiveness for her, and Allaah is Oft-Forgiving, Most
Merciful. 

With regard to the loss of her virginity as the result of
this zina, she is going to be faced with a serious problem, because if
someone proposes marriage to her on the basis that she is a virgin, he will
find out on the wedding night that she is not. If she tells people that she
has lost her virginity, then tongues will wag and she will bring shame upon
her family. If she keeps quiet about it, then she will be deceiving her
future husband. But I hope that this young woman’s repentance will be
sincere; if it is sincere then Allaah will give her a way out. 

With regard to him saying in the question that he cannot
marry her, I do not know why. If they have both repented and he can marry
her, that is good. If that is not possible, then Allaah will give her a way
out so long as her repentance is sincere, because Allaah says
(interpretation of the meaning): 

“And whosoever fears
Allaah and keeps his duty to Him, He will make a way for him to get out
(from every difficulty)”

[al-Talaaq 65:2]

This is general in meaning. And Allaah also says
(interpretation of the meaning): 

“And He it is Who accepts
repentance from His slaves, and forgives sins, and He knows what you do”

[al-Shoora 42:25]