How do you apologize to your girlfriend after sleeping with her best friend?

You can't really apologize to your girlfriend after sleeping with her best friend because she's lost trust in you, and her best friend. You shouldn't have done it in the first place. I've experienced this but actually I handled it really well. I made it so that my girlfriend and her best friend made me their enemy, attempting to save their friendship. If you're sleeping around with her best friend and most likely other people, then ask yourself, do you really want to be with the girl you were with? Just think about that next time, if there is a next time.