[Evanesco]

Hey,
The situation you're in really sucks. I know it hurts a lot when a crush doesn't like you back in the same way, and then even more when they love someone else. Like Jordan says, the fact that she's had sex with him shows that she's probably very invested in the relationship. Although best friends share a lot, you are perfectly within your right to ask her not to discuss things like sex with you. It can hurt, especially with your feelings for her.
It is difficult to move on from liking someone, but that would probably be the best thing for you to do right now. If you don't, there's the risk that this could drive a wedge between you as friends, and as cliched as it sounds, friendships are incredibly important. You don't need to move on by finding someone else to crush on, but try to acknowledge to yourself that these feelings won't go anyway, and focus on different things. Take time for yourself as well. Look after yourself, treat yourself, and perhaps try and spend some time with other friends or with a group of friends, to make the situation feel less intense.
Take care