I was only dating the guy I am engaged to now, and had developed feelings for another guy I met in college. I could never pinpoint the reason why we met, but he liked me too, even though I was (and still am) madly in love with my boyfriend. The college guy and I had a connection that came out of nowhere, and slowly, but surely, we built up a relationship that climaxed and ended in sex.
During the two semesters of the build up, my guy started figuring things out, and asked me if I was cheating on him with that guy. I denied it. I flat out lied because I just didn't want to lose him. We had already picked out my engagement ring in August. I felt like losing him would be the worst thing to ever happen me, but what it is really is to pray forgiveness from God and beg to be let into the Kingdom of Heaven. If I have asked God for forgiveness, do I still need to ask my fiancé for forgiveness too? I have repented, and have never done that again.
The problem is that you lied to your boyfriend, causing him to rely on false information. You repented of committing fornication, but what about your lying? A part of repentance is correcting past mistakes so that you are not profiting from your sins. When Zacchaeus repented on his sins, he offered to give back or give away any unjust profits that he might have made. "Then Zacchaeus stood and said to the Lord, "Look, Lord, I give half of my goods to the poor; and if I have taken anything from anyone by false accusation, I restore fourfold"" (Luke 19:8). In contrast, you want to profit from your sin by not retracting your lie.
If the fact that you were seeing and having sex with another guy while becoming engaged to your boyfriend brings an end to your relationship, then that is the penalty for sinning. Love is about commitment and you haven't shown that in your relationship with your boyfriend.