Made a HUGE mistake:(

So back in September one of my friends decided to have a camping trip for her Bday.  I took my boyfriend and everything was great. Until I got drunk and had sex with a random guy.   I felt Horrible, I cried and prayed over any one again and my boyfriend doesn't know. I want to tell him but I know he will leave me.  We just celebrated our 2 year anniversary. And everytime I look at him, I just want to cry because I feel absolutely terrible. I don't know what to do...  Shoulder I tell him and risk it, or keep it to myself?