Is the pain and sadness that the questioner feels, due to her inability to forget a married man she was previously involved with, considered a sin and a lack of contentment with Allah's decree?

Some scholars disliked the annulment of an engagement without need, but it is permissible by consensus if there is a justification, and the permission of the other party is not required. The woman whose engagement has been annulled should strive to forget the man and occupy herself with what benefits her, while seeking help through supplication and staying away from anything that reminds her of him. There is no harm in her offering herself for marriage to someone she approves of through her guardian (wali) or one of her mahrams.