How should a betrothed woman act who suspects she has lost her virginity without intercourse or an accident, and whose fiancé has given her the choice between separation or marrying her and then divorcing her after a period?

You made a mistake by telling your fiancé about these doubts. If he becomes disinclined towards you, then it is better to dissolve the engagement. You must be careful not to repeat this mistake. Do not tell a future suitor about these anxieties, as virginity can be lost due to many reasons, such as a strong jump or profuse menstruation.