I've had a simular feeling before, it's horrible. I didn't tell anybody because I didn't want to give up the one only thing that was somewhat normal in my life. Having two supportive parents provides that kind of illusion, that even though you have social anxiety, depression, or whatever, that some aspects of your life is still normal. We desperatly try to hold on to these things I think.
If she's your best friend, she should really know. Write her a letter, email or something personal and tell her your thoughts like you've written in your posting. Tell her you want to get it off your chest and that you're sorry for lying. They'll be happy that you're being honest. It will hurt both of you a lot more if she finds out accidentally.
Good luck!