I Had Sex With My Fiance's Friend, Please Help!

Please I am really scared and confused right now. I need serious advice.

It happened when I and my boyfriend broke up due to some misunderstandings, we were not engaged then.

We broke up and didn't contact each other for weeks. Then I traveled to Ogun state at my grandma's place for somethings. I met this guy and we got along well. We go on outings, had fun and eventually had sex. After that it dawned on me and I regret it. I ignored him for some days and later came to my grandma's place to look for me.We talked and he told me he want us to start a relationship. I declined and told him I have no feelings for him.

After a week I left Ogun state and came back to Lagos.

Afterwards I got back together with my ex and now we are engaged. We haven't picked a wedding date yet but we planned it to be next year.

Fast forward to last Month, my fiance told me he needed to rent an apartment for one of his friends coming to lag to stay permanently.
He got the apartment and the guy came to lag..
I didn't get to meet him until after a week...
I was in a mall with my fiance and he called him telling him he need to go out since he was bored. He came to the mall and behold it was this same guy I met in Ogun State. I knew I was doomed .... I got really scared and nervous at the same time cuz my bf never mentioned or showed me his pics before...

He was only looking at me without saying anything except to my fiance... I was even scared of leaving their midst cuz I was scared he would tell my fiance....

Later that week he called me with a different line and told me it was him. I ended the call immediately on hearing his name....

I can't tell my fiance what happened btw us or he would never forgive me...

The guy has been calling but am not picking his calls am scared of what he might say...

I told my friend abt it and she asked me to have an agreement with the guy never to tell my fiance but am really scared about everything and confused....

But then I remember have seen his face on my fiance phone before but didn't pay attention cuz he was on lowcut there...

Have been acting funny around my fiance especially when the guy is there and he is starting to suspect something is going on...pls help me...

I can't lose my fiance and I don't want the wedding called off...

Pls kindly advice but I can't tell my fiance cuz he often tells me he can't have a taste of what his friends have tasted...

I can't stop thinking and crying...