i have sync-ed my new i phone with my girlfriend's itunes library by mistake, how do I sync it with the new library I have created?

hello,

i have sync-ed my new iphone with my girlfriend's itunes library by mistake, how do I sync it with the new library I have created for myself?

I have created a new library for myself but when I click file > devices > sync "iphone" it doesnt do anything

I have erased all music on the phone itself via settings on the phone but cant now get it to sync with my new library.

d'oh!

Thanks

Mike