Cheated on boyfriend of 8 years with his best friend at our engagement party, how do I tell him without losing everything?

3 hours ago my best friend told me that my fiance cheated on me with his friends wife and the child she is currently pregnant with is his. I confronted my fiance and he told me that they were having an emotional affair but nothing physical had happened. <br><br>I'm 24 and he's 26. We have been together since high school and have known each other since we were kids so our love is real.<br>As I am very angry and hurt by his failure to disclose the truth to me and honestly his lying to me about the extent of the affair. Due to this I hooked up with his best mate who was consoling me at the time.<br>It was awful timing and I was very sloppy and I'm sure people will talk but I don't know how to tell my fiance what happened. We had an engagement party 3 days ago and I did it the night before in front of everyone.<br><br>UPDATE: I got home and my fiance was back from work and I tried to talk to him about the affair but he was very evasive so I told him I knew and he was taken aback. He tried to tell me how sorry he was and that he ended it but I told him I dont care and I dont know if I can forgive him. I told him I hooked up with his mate. He said I was lying but I showed him a video on my phone and he completely lost it. The Police showed up and I was taken away and he's seeking an immediate annulment and threatening to succesfully sue me for what I've done.