I had been involved in an extreme act of sin. I deeply regret for my sinful act and have been asking for His forgiveness and mercy. I was living away from my family for studies and found myself in circumstances when I was completely depended on a person. Both of us belong to families who practise religious values a lot. But I was in a crisis state and he helped me. I had respect for him and vise versa but it ended up like this. I have left studies and contact with the person or the environment.
I still long to have married the person when he asked me but I refused him as such a marriage would be unacceptable to our families. My family now wants me to marry a person whom they have appointed. I am still in state of confusion whether to marry the person without telling him all about myself. I cry a lot and feel I am on the verge of loosing my mind. I am not sure if one can be honest to a person when he is hiding things from him. Please guide me what I should do?

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. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The religion of Islam persisted in purifying the Muslim society and keeping the Muslim citizens chaste, so Islam forbade a man to be in seclusion with a marriageable woman, or to travel alone with her, or to enter to her without the presence of a Mahram (a non-marriageable relative) and other things that might lead to temptation, and fornication and/or adultery. Not observing strictly these matters always leads to afflictions and disasters. What happened between you and this man is the best example on what we mentioned, and we hope that this will be a lesson for every young person, male or female, and for their guardians, to take this matter seriously. Indeed you did well by regretting this act and we hope that you will sincerely repent; this applies as well to this man you committed fornication with. 

There is no harm for him to marry you, Inshaa' Allaah, if he repented to Allaah. If you wish to marry him, you have to endeavour to convince your parents; if they agree, then all perfect praise be to Allaah, but if they insist on objecting your marriage to him, then you have to obey them as it is an obligation to obey the parents in what is permissible, and marrying this specific man is not an obligation on you.

As regards the other man to whom your parents want to marry you off, if he is a religious man with a good moral character and you are able to fulfil the wish of your parents by marrying him, then this is a good thing as you are pleasing your parents by doing so. However, it is not permissible for you to inform him about your sin of fornication, rather you should conceal it. The Prophet said: "All my nation will be forgiven except those who openly expose their sins." [Al-Bukhari and Muslim] Even if we suppose that he asks you about this, you may use Tawriyah (saying something which has more than one meaning and intending a meaning different from what the listener is likely to understand). On this regard, and for more benefit please refer to Fataawa 90186, 89962 and 88652.

Not informing him about your sin does not mean that you are not sincere to him or that you are not having good marital relationship with him. This is something that happened and you repented for having committed it. So there is no religious benefit for him knowing about your sin.

This is of course if you wish to marry him, otherwise your parents are not permitted to oblige you to marry him as long as you are a pubescent person and in sound mind, and you are not obliged to obey them and marry him.

Allaah Knows best.