How can I repent, seek forgiveness, and find peace for my heart after being raped before marriage, while keeping the matter a secret from my husband who noticed nothing on our wedding night?

You are obligated to repent for what transpired between you and the young man, by feeling remorse and resolving not to return to such an act. You must conceal what happened and not tell anyone. Furthermore, engage frequently in good deeds, remembrance of Allah (dhikr), and seeking forgiveness (istighfar).