With regard to the past from which you have repented to Allah, may He be exalted, the fiancé or husband has no right to ask about it. And it is not prescribed for anyone who has fallen into sin to tell anyone else about it when Allah has concealed him, and he should not expose himself, Allah forbid.
If the fiancé or husband insists on asking, you do not have to tell him about what you did before you knew him. Hence you made a serious mistake when you told your fiancé some of what you had done. Rather he should only look at you as you are now, and if that suits him he should go ahead and marry you, otherwise he should leave you for someone else.
Now what has happened has happened, and you do not have the right to tell him any more than you have already told him or what he has found out himself. You can deny anything that others might tell him and that could damage your image and reputation.
But if he says he is going to divorce you after that, then this matter is in the hand of Allah and intentions may change. “You (the one who divorces his wife) know not, it may be that Allah will afterward bring some new thing to pass” [al-Talaaq 65:1]. So ask Allah to accept your repentance and to conceal your errors.
And Allah knows best.