It sounds like she's not in a good place. Chances are it has absolutely nothing to do with you - it's the boyfriend, the dog, she hates her job, her neighbor is pissing her off, her mom is complaining that she doesn't see her... whatever. You and the wedding just are another little thing that she knows she ought to be paying attention to and she simply doesn't have the energy.
So, don't get mad. The reality is that right now, for you, the single most important thing on the face of the planet is the wedding. But that doesn't mean anyone else sees it that way.
So, take the higher road. Stop and tell her that you love her, that you get that chaos is abundant and invite her out for a non-wedding, non-reality day. Have brunch, people watch, laugh at what the 14-year olds are wearing. She probably needs someone in her life to just support her right now. The same way you want her to support you.
And she'll get it. And she'll come around and you can pick out dresses and have a wonderful time and be back to the way things were.
Good luck. Keep us posted.