My [M] best friend [F] cheated on her boyfriend with me

This is gonna be really long so you can skip to ******* for sexy time. I need to add a bit of the background story here for context.

I’m currently 22 and my best friend, we’ll call her Kieran, is 21 and we’re still close friends to this day. So Kieran and I were best friends in high school for about 4 years at the time, we’ve been close since freshmen and we got closer over the years. I was basically this guy that she could always depend on for advice and help as she was a 4’11 feisty but amazingly cute chick who legit has one of the most amazing booty I’ve seen. She constantly had bad luck when it came to dating. She was very impulsive at times and the guys that she’d end up with are either emotionally abusive, or just pure shit heads. Of course, me being there I did what I could to help her move on and look for better type of guys but then we’re teens and we were pretty dumb at the time.