How should one deal with a fiancée who confessed to committing sins (short of fornication) with another person before the engagement, knowing that she has repented and started wearing the legal hijab, and there is a strong attachment to her?

If your fiancée repents sincerely of the sin she committed, then marry her. Otherwise, look for another righteous woman. She must repent to God and conceal herself, and not tell anyone what happened, for Allah is Sattir (the Concealer) who loves to conceal. And know that a fiancée is a non-mahram (definition) woman; nothing that was forbidden before the engagement becomes permissible until the marriage contract is concluded, except that the Shari'ah has permitted looking at what encourages one to marry her.