He said to his fiancée: If you conceal anything from me, you will be haraam to me after marriage
Question

My fiancé knows that I was in a relationship with someone else before I met him, and the other person is his friend. There took place between me and this person some things which did not go as far as major immorality, but they are haraam. Now I have repented and I ask Allaah to forgive me. 
The problem is that my fiancé is not sure about what happened in the past with his friend, and he has heard some of his friends say bad things about me and that this friend has told them what took place between us. My fiancé made me swear to him that I would tell him everything that happened and he himself swore that I would be haraam to him after marriage if I concealed anything from him or told a lie. I swore with the Mushaf in my hand and inside the mosque and upon the Qur'aan that I would be haraam to him if I concealed anything from him, but in fact I did conceal what happened to me in the past. Please note that I am going to get married soon, if Allaah wills, and I am afraid. Am I sinning with regard to him, knowing that he always tells me that he will never forgive me and will never be pleased with me before Allaah if I conceal anything from him. What should I do, may Allaah reward you with good?.
Praise be to Allah.Firstly: 
The fiancé or husband has no right to ask about his wife's past. It is sufficient for him that she is righteous and is known for good at the time of marrying her, and there is nothing wrong with her religious commitment or chastity. As to whether she did anything haraam in the past, but then repented from it and became righteous, it is wrong to ask her about that and make her choose between telling lies or getting divorced, or make her tell her secrets and disclose that which Allaah had concealed for her, then if she tells him the truth, that opens the door to doubt and suspicion. 
What some people call for, of each spouse being frank with the other and telling them of things in the past that Allaah has concealed, is wrong and ignorant. Rather they should be pleased that Allaah has concealed it and they should praise Allaah for it. 
Secondly: 
A wife or fiancée is not obliged to tell us what happened in the past that Allaah has concealed. Rather she must conceal herself, because the Prophet (blessings and peace of Allaah be upon him) said: “Avoid this filth that Allaah has forbidden. Whoever does any such thing, then let him conceal it with the concealment of Allaah.” (Narrated by al-Bayhaqi; classed as saheeh by al-Albaani in al-Silsilah al-Saheehah, no. 663). 
And Muslim (2590) narrated from Abu Hurayrah (may Allaah be pleased with him) that the Prophet (blessings and peace of Allaah be upon him) said: “Allaah does not conceal a person in this world but Allaah will conceal him on the Day of Resurrection.” 
If her husband or fiancé persists in asking, then she may use a double entendre, such as saying: Nothing happened between me and that man -- meaning that nothing happened today or yesterday, because she is enjoined to conceal it and there is no interest to be served in telling him. So it is prescribed for her to use a double entrendre, and in fact some scholars said that it is permissible to tell a lie in that case. For more details on that please see the answer to question number 83093. 
Based on that, we hope that there will be no sin on you because of the lie that you told, although it would have been better to use a double entendre. 
Thirdly: 
If a man says to his fiancée: You will be haraam to me after the wedding if you concealed anything from me, then she conceals something from him, she will not be divorced and no zihaar will take place, because divorce and zihaar can only take place after marriage, and the fiancé is saying this before the marriage contract has taken place. So no divorce or zihaar takes place. 
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: It should be noted that divorce can only take place after marriage, because it is the dissolution of the marriage bond. So before marriage there is no divorce. Even if a man says to a woman: If I marry you, then you are divorced, then he marries her, she is not divorced. Or if a man's wife said to him: I heard that you want to take a second wife and I do not agree to that, and she put pressure on him, and he said to her: Will you be pleased if I say that if I marry a woman, she is divorced? And she said: I am pleased with that, then he said it and did not get married, and if he did get married she would not be divorced, because that took place before the marriage. End quote from al-Sharh al-Mumti’ (13/7). 
We ask Allaah to accept your repentance and to set your affairs straight. 
And Allaah knows best.