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.
It is not permissible for a Muslim woman to have an affectionate relationship with a non-Mahram man as we clarified in Fatwa 81356. Therefore, it becomes clear to you that what you read in some websites that having sex before marriage is not forbidden is absolutely wrong and it is a statement by an ignorant person who does not know anything about Islam and does not respect Islam. Such statements are probably propagated by those who do not wish any good for the Muslim woman and want to make her fall prey to wicked men to corrupt her religion and violate her honor; so you should be careful.
Hence, you are obliged to take the initiative to repent from this relationship and from other sins that you committed with that young man. You should praise Allaah that you did not go to the extent of committing Zina (fornication) with him which is a grave major sin. Again, we stress the fact that you should cut this relationship with that young man and completely avoid him.
On the other hand, you should think good of your Lord as He forgives the sins; Allaah Says (what means): {But indeed, I am the Perpetual Forgiver of whoever repents and believes and does righteousness and then continues in guidance.}[Quran 20:82] For more benefit, please refer to Fatwa 87903.
As regards this young man marrying you, then you should think twice about it. Indeed, a non-Muslim would embrace Islam without him really willing to do so; rather, his objective would be to trick a Muslim girl and marry her. Once he would have achieved this, he would show her what he really is, then she would either be like him and become a non-Muslim or she would stay with him as a Muslim while he is a non-Muslim, and so, their sexual relationship would be nothing but Zina (fornication or adultery). It is also probable that dissension and disputes would happen between them and this might happen after she has children from him. Therefore, we advise you to be wise and rational and not to be affectionate.
However, you may seek the help of some trustworthy people to find out whether or not he has really embraced Islam and become a true Muslim. Then if it is predominantly thought that he sincerely embraced Islam, it becomes permissible for you to marry him and this marriage should fulfill the conditions of a valid marriage, with the consent of your guardian and the presence of two witnesses. For more benefit on the conditions of a valid marriage contract, please refer to Fatwa 83629.
However, if there is any doubt about his becoming a Muslim, you should avoid him, and Allaah may facilitate for you a marriage with a better husband. For more benefit on how to treat love sickness, please refer to Fatwa 84544.
Allaah Knows best.