IIN that I don't want to tell friends/family I'm no longer Christian?

It's not shying away, you have a right to privacy, who ever said you have to share absolutely everything with everyone? I don't know if it's of any comfort, but try to imagine throughout history how many people have been in your place around the world, maybe you'll get the feeling like you are not alone. My advice is to try and make it on your own as soon as you can, and not to tell your family until then. I think it's completely useless to try and change people like that, the only reasonable thing is to try to make them accept you in time. But for that you need to be independent, otherwise you'd just be starting a purposeless war.

Regarding your friend, I think if she's a true friend she should be OK with it, although perhaps not right away. It all comes to whether you think she'll tell on you or not. Maybe a good way to go about it is gradually, like starting to show little by little how you feel with some subtle comments and seeing how she reacts, i don't know.