Bottom line is you're upset because of the way that she handles things. Can't do anything about it, you either deal with it or leave. I'm not being an asshole, I genuinely feel sorry for your situation. Because while everyone else is arguing with you, I'm going to be the only one to say that, yeah, this is shit, and it's double shit because it's not something you can change her mind on. This is how she is, it sucks that there is a disconnect in this department. But, try to concentrate on the fact that everyone is different, and while this kind of thinking is unromantic, depressing, whatever adjective you want thrown in, you're seeing it from your framework of understanding. This is probably normal for her. The only silver lining I can give you is this: her way of doing things is not evidence of the same thinking and reasoning that you would have to exist in order to reach a point in which you are doing things/ acting in the same manner

In simpler words: For you to be thinking/ acting what how she's thinking/ acting, you'd have to have doubts in the relationship etc etc. For her to be thinking/ acting like this, nothing bad had to happen as a precursor, this is how she operates.

Side note: There is a high chance that this kind of thinking is a statistically good indicator of ones flippant-ness, though. So that sucks. All the best man, I hope this works out for you.