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 is His slave and Messenger.
Zina (adultery or fornication) is a grave major sin and it is an obligation to take the initiative to repent from it. In Islam, there are no specific rituals for performing repentance. Nonetheless, there are some particular conditions if you fulfill them, your repentance would be accepted, Allaah willing. For more benefit, please refer to Fatwa 87903.
Furthermore, in order to sincerely repent, you are obliged to avoid that young man. Indeed, feeling sad because of being far from him is not a proper feeling. In the future, you are obliged to avoid establishing any relationship with non-Mahram men, or talking to them via Internet chat or by any other means. Also, we advise you to keep company with righteous women and attend circles of knowledge and lectures.
However, if this young man repents, then it becomes permissible to ask Allaah to make him your husband, and there is no specific wording for such a supplication.
In order that the supplication would not be a reason for you to be attached to him, and probably commit Zina again, you should rather ask Allaah in general to bless you with a righteous husband, whether it is this young man or anyone else.
Allaah Knows best.