How do you get over your boyfriend sleeping with your best friend?

You treat it like what it was a one night stand. You cant blame him for not yaking it further and i am sure you are both adults so you have to act like it. By not holding your self or him responsible for taking it further "relationsip" you'll free yourself of feeling empty. If you act casual and like it never happened he'll be o.k. eventaully too. He'll see that you are cool and so they will be the same. In the future the best bet is not to have casual sex with friends, this can lead to losing friends and empty feelings of your own. You should not have casual sex with people if you want to form emotional binds with them. Wait until you are in a relationship. Good look, i have slept with a few of my friends and since i act like i dont care they are cool about it to. Please use protection, friend or not, stuff is out there and you have to protect yourself.