so it has never terminated naturally?Both times I broke first and put her in a crate
I have read other people's posts about hens dying on the nest when brooding, though it is the sort of behaviour that natural selection should weed out quickly. Sometimes it turned out that the hen was actually ill, not broody.
In the case of your young broody, it sounds as if it's definitely broodiness not illness, but I would be inclined to just leave her be, and wait for her hormones to switch off themselves. It may be tough to watch, but trying to switch them off by breaking her isn't working - hence this thread, of course. Her will to survive should be stronger than her will to reproduce, and if they switch off naturally I would expect them not to resurface so quickly. Hens build reserves before they go broody and she must have spent a lot of hers by now. It shouldn't take too long.