Is a woman who is engaged (to be married) obligated to tell her fiancé the details of her past relationships with young men after she has repented, fearing that her situation may be exposed and she may be disgraced?

If you repent sincerely from the calls, then rejoice in the acceptance of your repentance. If the suitor asks you about them, do not tell him, for concealing one's sins is legally مطلوب (desirable/required), and a penitent person is not punished for the sin from which they have repented.