I made a mistake. Should I tell him? Options

[starsandmoons]

So I've been friends with Guy X for about 4 years now and 2 1/2 of those years I was deeply infatuated or in love with him whatever you want to call it. He asked me out at one point but he was just out of a relationship and I didn't want to be the rebound. It just wasn't the right time. After a week I ended up saying yes and by that time he had decided he only saw me as a friend. For 4 years we were best friends. The closest we could get. It was amazing the type of relationship we had I could tell him anything and everything and he would do the same with me. He moved away and our relationship suffered. We were still close but not the same.

Then I met Guy Z. He was everything I could hope for in a guy. Sweetest guy, but he was deeply scarred from his past relationship where his girlfriend cheated with her ex-boyfriend of a year or so and he caught them and later she turned lesbian. Nonetheless he was very carefully with me. He wasn't as attached or attentive, which kind of made it hard on me. Sounding much like a soap opera. It gets worse. Guy Z and I were together for about 7 months when one of my best friends invites me over to a sleepover. Guy Z has just recently started talking to his ex again and I'm jealous nonetheless because he starts lying to me about it.

The sleepover. Guy X is there and he is very persistent in his flirting. I constantly push him away, but eventually I let him. We made out. A lot. I... never imagined myself doing that. Things kind of got out of hand and the sad thing is Guy Z was on my mind the entire time. My boyfriend... I love him. Guy X and I had never had even kissed or anything before this even when we "liked" each other and now at the worst moment when I had finally gotten over him and was happily taken, he does this. I did this. I cheated. I don't want to lose my boyfriend, but I feel like I need to tell him, then again telling him could mean the end of 8 wonderful months. Guy X means nothing to me. We're not talking anymore. I've completely detached myself from him. He told one of my friends who then questioned me about it trying to pressure her to tell my boyfriend. A mess.

What should I do?