I fell in love and slept with my best friends girl and I feel awful
Why did you do that and what do you think will happen?
I did it because I am so attracted to her, but they might break up over it.
That is your best friend's girlfriend and messing with their relationship will probably cause problems for you in his eyes.
I don't care what people think of me.
You may care when your reputation gets around and you find it hard to get to know people.  You need to apologize and make things right.