Oh dear. You have my sympathy Hecate, 'tis very frustrating losing your work like that. I suspect the simplest solution would be to have a confirmation dialog come up if you hit the cancel button, so you can cancel your cancelling (does that make sense?) if you hit it accidentally. Since on Talk the buttons are Preview/Post, having Send/Cancel on a different form is rather counter-intuitive.
It won't be a safeguard if you close the window or click on another link of course, but hopefully it'll ease the problem a bit. Some browsers have add-ons that allow you to recover form data, which you might find handy as I'm sure MN isn't the only place that has this problem.