How do i break up with him nicely?

I am engaged to a beautiful man, but unfortunately i got cold feet and engaged in an online relationship with another. This relationship is platonic however the other person knows that i am engaged and cares for me overly. i have come to see this was a mistake and want to end things with this person but i'm scared of hurting his feelings. also he lived in another country. what do i say to let him down gently?