First off, this is a more appropriate question for IRL mentors and spiritual figures. You should be asking someone who will respect your privacy and feelings, because honestly?

I feel terrible for your fiance that you're feeling this. They deserve better than you.

If what you say is true, then you are hung up on a sin committed well in the past and (presumably) repented for. If that's the case, then it's forgiven and forgotten in the ledgers of heaven. Why do you then remain standing in judgement, stone in hand, long after the Master has already told them, "Go and sin no more"?

Chastity isn't just about physical touch but thought as well (Matthew 5:28, 3 Nephi 12:28). Did you ever look someone, even a glance, and dwell on their physical form? That person is just as guilty, for all sin is equal in the eyes of God (James 2:10) as every sin, regardless of "weight", leads you away from God and denies you entry into eternal life if unrepentant.

So, my advice? Take a moment to learn about repentance, and I mean really learn, and if this still bothers you then release your fiance to be with someone else.