Please help/ end of relationship

Hello Glowfriends,

I have a confession to make. My boyfriend and I broke up two weeks before thanksgiving and I slept with someone he knew in the meantime. (My ex boyfriends' brothers' best friends brother). My ex-boyfriend and this guy recently met, maybe a year ago. Not only did this dude have a huge crush on me, he went so far as to telling my ex-boyfriend, when he was my boyfriend at the time. After having sex with him I immediately regretted it, and hadn't stop crying/thinking about it ever since! I've been praying about it but I'm still so broken. I feel like this would crush and ruin my ex boyfriend if he knew. We have recently spoke about getting back together, but he doesn't know about this situation and it's killing me! Please someone help me :/