My past is ruining my relationship!

I "foolishly" told my fiancé about a sexual encounter I had, ok 8 YEARS AGO! We have an open relationship & he asks me if I ever had a 3some? I told him a 4some, 2 guys & another girl. He is obsessing over it. Wanting exact details, what position who was where etc. It's gotten soo bad. He puts on a porno & asks me.. Did that happened? Did this happen? Then we fight about it, he calls me a skank etc. it's crazy, because he wants a play by play, & he thinks I'm lying when I tell him I really don't remember everything! I was drinking & on Ecstasy! Not to mention it was 8 yrs ago! This is really destroying our relationship! We won't talk for days, then he threatens me to give him details or I'll be sorry!! Whatever that means? I'm so unhappy, I don't know what to do?  UPDATE:THEN HE JUST TOLD ME NOW, DONT TALK TO HIM UNTIL I GIVE ALL DETAILS!! Btw I cnt just leave him bc we just had a baby 3 months ago!!! Totally stressful!!! I have to even describe the exact lingerie I had on!! Twisted!! CHECK OUT THE POST HURT & Confused!!! I wrote that too!! So he. Has a lot of nerve!!! Bc look what he did to me!!!!!