Hey, I know it's hard to appreciate when you're angry with your mother, but she really just wants the best for you. They did a lot of research on the effects of corsets and it's now established that they do a lot of harm if someone wears them long-term. And I know that you only wear them for a couple of hours, etc, but you've got to understand the background behind it - in your grandmother's day ladies were expected to have tiny waists, and those who didn't got a lot of flak. Your mother's day would have been much more enlightened as they were told that they didn't have to have tiny waists - everyone accepted that everyone had normal shaped waists. So can you see that, by wearing corsets, to your mother it may seem as if you're almost reverting to a time which was more repressed? It's a tender issue; it might seem ridiculous to you, but it really was the attitude at the time.
It's like your daughter coming home and saying that she's decided not to go to uni because only boys go to uni, that sort of thing. I know it may seem a bit in-depth, but this is perhaps how your mother is feeling right now.
Anyway, hope you work things out together!