I messed up, and I don't know what to do.

Dear Sacred Posts,

I need to get this off my chest. I feel like I'm drowning in guilt, and I don't know who to turn to. It's about my best friend, Liam. We've been close since kindergarten, practically brothers. He's always been there for me, no matter what. I messed up. Badly.

Liam's always been unlucky in love. He's a great guy, but he gets nervous and awkward. He finally met someone amazing, Sarah. They were perfect together. I could see how happy she made him. I was happy for him. Really, I was. But then… I started spending more time with Sarah when Liam was working late. Just friendly stuff at first – grabbing coffee, watching movies. But then one night, after a few too many drinks, we kissed. I swear, I never meant for it to happen. But it did. And it felt… good. Too good.

We both knew it was wrong. We both said it wouldn't happen again. But it did. A few more times. Now Liam is planning on proposing, and the guilt is eating me alive. He thinks Sarah is perfect, and she thinks I'm a great friend. I am not. I'm a terrible friend. A terrible person. I haven't told Liam. I don't know if I can. I'm afraid of losing him, of ruining everything. But keeping it in is killing me. What do I do?