Friendship issues; need advice?

i used to be very tight with this girl, she changed schools, now she barely keeps up with me and it seems as if she is avoiding me. i write on her facebook and she doesn't acknowledge it. she ignores me, it seems. it saddens me to NO END and makes me in fact feel very depressed. she causes me to lose sleep and be very adjetated and unfocused. and when she does talk to me it makes me nervous too which is so silly because that's what i want... overall she just makes me so nervous! and i haven't worked up the courage to tell her, what should i do to make myself feel better?