I cheated and I can't live with it

Okay, I need to get this off my chest. I don't know who else to tell. Maybe nobody. But it's eating me alive. I cheated on my boyfriend, Mark. It was a one-time thing, a stupid drunken night with a guy from work. His name is Dave. I barely even know him. We went out for drinks after a project wrap-up, and one thing led to another. I don't even remember all of it, just flashes. Waking up in his bed. The shame. The instant regret.

Mark is… he's the best guy I've ever known. He's kind, he's funny, he's patient. He's everything I'm not, I guess. I don't deserve him. That's the thing that keeps running through my head. I DON'T DESERVE HIM.

I haven't told him. I can't. It would destroy him. He trusts me completely. He thinks I'm this good person, and I'm not. I'm a liar and a cheat. I know some people would say I should tell him, that he deserves to know. But what good would it do? It would just hurt him for no reason. It was one mistake. One stupid night. I can't take it back.

But the guilt… God, the guilt. It's crushing me. Every time he looks at me, every time he tells me he loves me, it's like a knife twisting in my gut. I feel like I'm living a lie. And I don't know how much longer I can keep it up. I keep thinking about telling him, just to get it over with. But then I see his face, his trusting eyes, and I can't do it. I'm a coward, I know.

I started going to church again. I haven't been in years. I don't even know if I believe in God anymore, but I needed somewhere to go, someone to talk to. The priest just told me to pray and ask for forgiveness. But what if I don't deserve forgiveness? What if I'm just a bad person?

I don't know what to do. I feel so lost and alone. I just needed to tell someone, even if it's just a bunch of strangers on the internet. Maybe someone out there has been through this before. Maybe someone can tell me what to do. Or maybe I just need to keep carrying this burden until it breaks me. I don't know. I really don't.