Should she tell her husband about her previous relationships?
Question

                I am a young woman who has received a proposal for marriage from a man who has all the qualities that a Muslim girl could hope for in her life partner, praise be to Allaah. I accepted and the nikaah (marriage contract) was done a short time ago. I do not see anything but good in this young man, but the problem is that before he proposed to me, he asked my friend about me, and she told him that a long time ago I had a relationship with another young man, but the relationship ended and I repented to Allaah. I only found out that she had told him about that a short while ago, and I bear witness that I have repented to Allaah and come back to my senses, and I stopped speaking to that man. But I got upset when she told me about that and I shouted at her, but she told me that it was her duty to tell him, because it is the matter of a lasting relationship and I have to be frank with him about that. My question is, do I have to tell him frankly if he asks me? Or should I conceal it? I am afraid that this past will destroy my life.
Praise be to Allah.Your friend made a mistake by telling the one who came to propose marriage to you about your previous relationship from which you have repented. What she did is not good and is not in accordance with sharee’ah, nor is it wise.  The Muslim is enjoined to conceal his brother’s faults if he sees him committing sin in secret, so how about a sin from which the person has repented?! 
What she did is not naseehah (sincerity or sincere advice) which is enjoined by sharee’ah on the one who is asked about a person with a view to marriage because that has to do with what he knows of the characteristics that are present at the time of asking, and it is not permissible for anyone to mention a bad past from which the person has repented. 
What we understand from your words is that your friend told your husband  of your previous relationship before he proposed to you. This indicates that he has forgiven you for that because he knows that you have repented and become righteous. 
He did right by doing so, for there is no one who does not make some mistakes, but if he repents from them, he cannot be blamed or punished. The Messenger (peace and blessings of Allaah be upon him) said: “The one who repents from sin is like one who did not sin at all.” Narrated by Ibn Maajah, 4250; classed as hasan by al-Albaani in Saheeh Ibn Maajah. 
What you should do is avoid bringing this subject up with your husband, but if he brings it up you should tell him that it was a passing relationship and one of the tricks of the Shaytaan, but you regret it, and Allaah has guided you and enabled you to repent. 
Do not worry about this past so long as you have indeed repented and become righteous. Ask Allaah for strength and guidance and to accept your repentance.  
“And verily, I am indeed forgiving to him who repents, believes (in My Oneness, and associates none in worship with Me) and does righteous good deeds, and then remains constant in doing them (till his death)”
[Ta-Ha 20:82]
The Messenger of Allaah says: “Every son of Adam is prone to err, and the best of those who err are those who repent.” Narrated by al-Tirmidhi, 2499; classed as hasan by al-Albaani in Saheeh al-Tirmidhi. 
We ask Allaah to bless you and bring you both together in goodness.  
And Allaah is the Source of Strength.