How could they? My boyfriend and my bff

Dear Cupid... Help!

My boyfriend & I are both 25 and have been dating for 4 years. Our relationship was one filled with so much laughter and joy - my best friend. It was a healthy relationship from my perspective and through the perspective of others (friends, family etc) everyone was rooting for us.

Here comes the plot twist. There was a period of time that I travelled for work. There’s was one assignment I had where I was going to be gone for a month straight. This occurred shortly after we moved to a new city. I sent my boyfriend the number of my best friend in the city...let’s call her Megan. Megan and my boyfriend had met several times through me and I thought it would be good if they could hang out while I was gone because neither of them had many friends in the new city because we had not been there long. 
Megan came over last night & told me that she and my boyfriend went out when I was traveling for work with a group of people which I knew about and was cool with. She then went on to say they got very drunk and slept together. This all happened a year ago and she just told me now. I confronted my boyfriend too and he admitted it. They both hid it for a year.

My boyfriend and I had never had any issues about trust or cheating or anything like this before. I truly thought I was going to end up with this man and before yesterday nothing would have made me happier. What do I do?

I am going to be straight with you here.. I do not believe that he would have ever told you about him cheating had he not been confronted with the truth. This leads to the question would he have kept it from you forever? It will be HARD, but for your sake, you need to move on. How he wasn't beside himself with guilt I will never understand & I hate to say it but this may have happened on more than one occasion and it only came to light because your 'friend' told you. I never believe cheating to be a mistake, it is a conscious decision, but he may argue he was drunk. He, however, was not drunk for the past year (I assume) and therefore consciously lied to you the entire time! If you stayed together I can guarantee you would never trust him again and this would cause you to constantly worry. You deserve a lot better.

This says so much about them as people and you truly have no reason to ever talk to either of them again. They made a terrible choice in betraying you.