Unfortunately, I don't think so. I find that its hard to get through to us in general because its our thoughts/feelings and we are dealing with them constantly. So even if you knock some sense into her for that moment, she will always revert back to what she was initially thinking/feeling. Its embedded in her right now. She'll have to get over it on her own.
BTW, her words scare me. Personally I find its hard for me to adjust to change...well, change that's not my own. I guess its the idea of a man proposing and wanting marriage so it's kind of his idea? Idk. Parts of her may seem a bit out of control in this.
Who set the wedding date? Have you suggested it being pushed back to ease her mind? Idk.