The guilt is eating me up

I messed up, big time. My best friend, Lisa, she's always been there for me. Through thick and thin, boyfriends breaking up with me, family stuff... you name it. She's the best. And I slept with her boyfriend, Mark. I don't even know why. I was drunk, he was there, and I guess I thought he was cute? It was a one-time thing, but that doesn't make it okay. He swears he won't tell her, and I'm supposed to just pretend it never happened. But I can't. Every time I see her, I feel like throwing up. She's so happy, and I'm ruining everything. I should tell her, right? But I'm so scared of losing her. I don't know what to do. I hate myself right now. I wish I could take it back, but I can't. Please, someone tell me what to do.