I still feel guilty about it

Okay, so I need to get this off my chest. It's been eating me up for years. When I was like, 16, my best friend Sarah had this crush on Mark. Mark was... okay. Not my type, but Sarah was head-over-heels. She talked about him CONSTANTLY. Anyway, one night we were at a party and Sarah got super drunk. She asked me to watch Mark for her, make sure no one else made a move. Well... I made a move. I kissed him. And he kissed me back. We didn't go any further, but still. I told Sarah that Mark spent the whole night talking to another girl, this total lie. She was heartbroken. I felt awful then, and I still feel awful now. We're not even friends anymore, we drifted apart after high school. I just... I wish I could take it back. I was so jealous of her getting all the attention, and I ruined something for her. I'm sorry, Sarah, wherever you are.