I cheated, and I don't know what to do

I messed up. Big time. I've been with Mark for almost five years. We're… we're good. Comfortable. We know each other inside and out. We have a routine. Maybe too much of a routine. He's a good man. Honest, kind, dependable. Everything I ever thought I wanted. But… I cheated. With a guy from work. His name is David. He's… different. Exciting. Kind of a bad boy, I guess. The kind I thought I was over.

It started innocently enough. Late nights at the office, working on a big project. Coffee breaks, shared frustrations, a few laughs. Then a drink after work. Then another. And another. He made me feel… seen. Like he actually noticed me, the real me, not just Mark's girlfriend, or the girl who does the laundry and makes the dinner. He paid attention. He listened. He made me laugh. I know it sounds cliché. I hate myself for it. It happened a few weeks ago. One night. We went back to his place. I don't even know how it happened. One minute we were talking, the next… well, you know.

I haven't told Mark. I can't. I don't want to hurt him. He doesn't deserve this. He's done nothing wrong. But the guilt is eating me alive. Every time I look at him, I feel sick. Like I'm wearing a mask, pretending to be someone I'm not. I keep replaying the night with David in my head. Part of me feels… alive. Like I woke up from a long sleep. But the other part… the bigger part… feels disgusted. With myself. With David. With the whole situation. I don't know what to do. Do I tell Mark and risk destroying everything? Do I keep it a secret and live with the guilt? Can I even do that? I don't want to lose him. He’s my best friend. The thought of him finding out some other way… or worse, me eventually blurting it out during an argument… makes me want to throw up. David keeps texting, wanting to see me. I haven't replied. I don't want to see him again. It was a mistake. A stupid, awful mistake.

I feel so lost. I used to think I knew who I was, what I wanted. Now… I don't know anything. I just want this feeling to go away. I want to be happy again. I want to be the person Mark thinks I am. Maybe that person never existed. Maybe I'm just a terrible person. I just needed to get this off my chest. Thanks for listening, or reading, or whatever. I feel a tiny bit lighter, but not much.