T.S.
I think it may make you feel better to know that ALL girls this age (and beyond) go through this to a varying extent, not just the shy ones. I have seen the most happy, outgoing girls shut down and withdraw, just because they got dissed by a friend, or two :(
What you are telling her is great, but also try not to let her dwell on it too much. Hear her when she needs it, empathize and then change the conversation to the positive. So sounds like Sally's being a pain today, who else did you talk to? Did anything fun or cool happen at school? Keep her as busy and engaged with activities and as many other friends as you can. The busier she is the less she'll focus on one or two friendships, and the happier she'll be :)