I am in a relationship with a wonderful woman. We are engaged to be married. I am a man of God and I am very religious. Recently one of my ex-girlfriends contacted me and wanted me to offer some sexual services over the phone. I felt it was wrong but I was stupid and went along with it. We got started but then I stopped and said I couldn't continue. It didn't feel right and wouldn't be fair to my fiance. So I hung up and prayed and asked for forgiveness. I feel really guilty about it and I don't want any part of that anymore. Should I tell my fiance? Is this considered adultery and lust? And finally will I be forgiven?
All sins can be forgiven (II Peter 3:9). Why do you think you would be treated differently from anyone else?
What you did cannot be called adultery because you are not married. The biblical terms would be lewdness, lust, and uncleanness. "Let us walk properly, as in the day, not in revelry and drunkenness, not in lewdness and lust, not in strife and envy. But put on the Lord Jesus Christ, and make no provision for the flesh, to fulfill its lusts" (Romans 13:13-14).
What you did was wrong, but you've repented of it. It was a sin against yourself (I Corinthians 6:18). It doesn't impact your fiancee's future, so there is no reason to bring it up unless you are wanting to cause a fight with her, make her not trust you, and possibly call off your marriage. If ever asked, you don't lie about it, but I would strongly suggest that you don't get into details. She doesn't need to know when or with whom. What would be most important for her to know if you are asked is that you regret it, you stopped it, and you have repented of it.