OK, I've calmed down and spoken to both of the committee members, and it's not so bad after all.
First, I emailed the one who wrote the email and said of course I'd love to meet with her, as I hadn't realized I was so far off-base. She wrote back and her second email was less harsh and more understandable.
Then the chair emailed me to say she'd spoken with the other member, and I wasn't so far off base, but there were things that needed to be included, and she was confident I could do so, and she just wanted me to have another perspective. Basically, she saw how the feedback was presented to me (she'd been copied on the email) and wanted to reassure me that it wasn't a bad thing.
It doesn't help, of course, that I still sometimes have this bit of insecurity that comes up from my past. It's the 'former welfare recipient who has the nerve to think she could actually be a Ph.D.' insecurity, and I can't seem to completely wipe it out of my psyche, although I continue to work on it.
Now, the thing is -- and this is where it gets embarrassing -- I am, in fact, missing something very important in my proposal. Here I am teaching policy analysis, having done it for a living (and as a volunteer), and I became so myopic about this dissertation that I couldn't seem to recognize that this was indeed a policy analysis. There are certain elements that need to be present in an analysis, and they aren't there in my proposal. So the prof was right on, just a bit harsh. I'll meet with her later this month, and hopefully I'll have some work done by then and can run it by her.
Anyway, I mentioned my embarrassment to my chair, and her response was:
Don't worry about it - this is why you work with other people on a research project. It is sooooo easy to miss something. Experts do it all the time.
Gotta love a chair like that, dontcha!