Sounds like a very stressful mess...
Do you think that you could try to "chill out" a bit and then take deep breaths and approach her about the whole thing?
In other words, if you can get to a point where you think you can broach the topic without screaming or cursing her out, then I think you might want to tell her exactly what you wrote: "I get more hurt every time I look at her, the way I feel she's betrayed me. What's worse, she acts like nothing at all is wrong. I don't know if she's so obtuse that she doesn't realize how much she's hurt me, or she just doesn't care. I should add that she knew when we moved up here, away from all our family and friends, that she was one of the few people we knew. I have essentially spent the summer alone and friendless."
