I slept with my best friend SarahJay's (SJ) boyfriend (I'm a guy), she hates me, what to do?

Okay, well, i don't know why you slept with him, but it seems that you did it just to prove a point which isn't a good idea. Give her some time.If you've been friends for so long.This wasn't exactly helping her out though. Sure he shouldn't have dragged the situation on like he did but you trying to expose him like you did was the wrong way to go about it. You should've tried maybe once or twice and let him do it. I mean, all i can say is to hope that she forgives you and gets over her anger because you've done the damage and all you can do now is wait for it to heal