Not only did I invade his privacy, but I slept with his best friend.
During your relationship, how did you feel?
Well, I didn't really care about him
It's wrong to invade someone's privacy and cheat on them. If you didn't care about this person that much then you should've broken up with them.
Well, maybe it was a mistake but it was fun.
Cheating is wrong, but add that to invading their privacy is worse. Break up with a person if you must, just don't violate trust, as that is gross behavior.