What is the ruling on my five-year relationship with a non-Muslim Filipino nurse, and should I tell my current fiancée about this relationship, or should I leave her and go back to my previous girlfriend?

This emotional relationship must be immediately severed, and sincere repentance must be made, for the girl is a stranger (non-mahram), and it is impermissible to continue. Leaving her is not an injustice to her, and her claim of unforgiveness is baseless, as she was not forced into this relationship, and you are not obligated to marry her.

Initiate the completion of your marriage to the betrothed girl, for it contains many legitimate interests, such as chastity and progeny. It wards off the perils of desire, increases offspring, alerts to the delights of the Hereafter, fulfills Allah's will for the continuation of the human species, obeys the command of the Messenger of Allah, peace be upon him: "Marry and multiply," and ensures remembrance and raises ranks through the supplication of a righteous child.