I ended up cheating on my girlfriend with her best friend and she caught me.
Why would you ever cheat on your girlfriend with her best friend?
I want to prove that she's a loose slut and that it's her fault for leaving me alone with her best friend.
You should never hurt someone that cares about you. You disrespected your girlfriend and your relationship by cheating on her. There is a high chance that you will experience the same hurt that you caused her.
It's fine. I'm feeling good since I managed to hurt her with all this.
All proving something about her best friend did was caused you to cheat on her. If you really wanted to try that plan out, than why not find someone who is single instead. It sounded like all of this was almost premeditated.