How do I re-teach my horse to spook in place?

My mare seems to know how to spook in place. She will do it when it's too hot to move much or when she's tired. Lately she started spooking by cantering away. Ok, it did get very cold suddenly and she's been stall-bound because the fields are completely flooded but still.

I'm most probably doing something wrong. Should I shut her down harshly and immediately? I've been trying to stay out of her face generally, so when she spooks I try to stop her gently, with my seat and a little bit of hand. It's now starting to get on my nerves and I don't think it's good for her training to be allowed to just do whatever she pleases, even if I do get harsh with her.