Is it obligatory for a girl to inform her fiancé of the details of her previous relationships, after he has found out about them and intends to divorce her after marriage?

A suitor or husband has no right to inquire about a woman's past from which she has repented, and a woman should not expose herself with what Allah has concealed for her. You made a mistake by telling your suitor some of what you had done. You are not obliged to add to what you have already told him, and you can deny anyone who tells him anything else offensive. The matter of your divorce is in the hands of Allah, and intentions can change, so supplicate to Allah to accept your repentance and to continue His concealment and well-being upon you.