I slept with my friend's girlfriend
Is there something that led you to sleep with her?
Yes of course, I thought she's easy.
Part of being a good friend is being loyal and understanding. Sleeping with a woman that your friend is dating is wrong since it is cheating and will cause a lot of emotional distress in your friend. You should apologize to your friend for what you have done and ask for forgiveness.
Why would I fix my mistakes? I'm not bothered by this.
It sounds like you've just given up on your friendship. It's never good to lose a loyal friend.