Is it obligatory to go and speak to a girl and seek her forgiveness years after wronging her by dating her fiancé, knowing that there are fatwas that permit suffices with repentance without seeking pardon from the person?

You must repent to Allah for the aforementioned relationship by abandoning it, regretting the act, and resolving not to return to it, while concealing yourself and not publicizing the sin. What happened is an injustice to yourself and disobedience to Allah, which necessitates repentance. If you caused the disruption of that woman's engagement through that forbidden relationship, then seeking forgiveness for her is sufficient for you, and there is no need to ask for her pardon or inform her of what happened; rather, you must conceal yourself.