Let's native speakers correct me if I'm wrong, but, probably it's common to many of European languages metaphor for 'giving up', 'changing one's mind under the pressure', the pressure being either external (the guy, her circumstances) or sometimes even internal.(her own emotions).

That sentence is one of the ironic point of vampire tales, as guy must be vampire and his girl friend is ordinary human being. like story of twilight and many other. Vampire guy must be very solidarity without her, but there is some misunderstood between two of them. he trying to convey his wholeheartedly love to her,asking for break rule of Vampire and human world, or breaking her blind superstitious, which bond with fear and hatred toward him. it just my thoughts, it can be mean another way... hope you can find some useful...

thanks a lot, Mike.　so.. Please say she'll break means boy want girl to allow his temptation?

Please say she'll break, (give up. all him to be with her.) in this case, she keeps denying him and him "breaking" her would be allowing him to go home with her (sexual) for the night.