I think Jadis had used magic so long in her life on Charn, and for so many hateful reasons, that when she got to the WBtW, where magical powers don't carry over, she literally could not live like a normal being without power, without hate, without the urge to dominate. (Rhymes.) So she felt like she was dying. The other characters, who did not hate that level of hate, just felt at peace and at stillness because they were in a liminal space.

Posted by: @cobalt-jade

I think Jadis had used magic so long in her life on Charn, and for so many hateful reasons, that when she got to the WBtW, where magical powers don't carry over, she literally could not live like a normal being without power, without hate, without the urge to dominate. (Rhymes.) So she felt like she was dying. The other characters, who did not hate that level of hate, just felt at peace and at stillness because they were in a liminal space.

Wow, what an interesting answer!  I think you're on to something there.  You know, when I try to imagine a world of perfect peace, like the one we Christians believe Jesus will  bring about someday, part of me is afraid.  What will it be like?  What will we do?  Will I miss this/that person or this/that thing?  Part of me dreads the thought of it, as nice as it sounds on the surface.  Ultimately, I know in my heart of hearts that it will be awesome beyond anything I ever dreamed of.  My Lord will be there, and my family will be there, and I believe I have a lot of amazing surprises to look forward to up there. But on this side, here in the "Shadow Lands", it's a bit disturbing, if not terrifying, an unthinkable unknown so extremely different from my own world.  I'm sure I'm not the only person who has ever felt that way, and not just Christians.  A place that's all peace, all the time, sounds great at first but a bit freaky if it's forever. 😀

So returning to the question, I'm sure someone like the Witch, who arguably had more dark magic power than any other character in fiction, experiencing something like that must have been something like that.  Her power was useless, if she even had it anymore (we know she didn't in our world, lol), and there was nothing she could do to change her circumstances.  Someone like her, super-powerful, finding herself suddenly powerless - I guess it's kind of a common-sense thing when you think about it like that.

I had recently done a topic about the Powers of Jadis. I got a few responses saying that if she was in another world or a different place from Charn, she may have lost her power. In this case, I think it's because she was in a different setting, she may have had a different affect, especially considering she had powers and Digory and Polly didn't.  Like @cobalt-jade said, since she had lived so long with powers, the sense of not having them took her away completely, almost as if the power that she had is what she was made of.  

I was watching the thing "Into the Wardrobe" on YouTube, and it was very interesting. He just touched on the origins of Tash, and one theory was that he may have originally been from Charn. Jadis could have allowed herself to be possessed by Tash... it would have explained a lot about her superhuman strength. When she was losing her beauty and strength in the Wood between the Worlds, the argument could be made that Tash could not dwell in the Woods between the Worlds, because evil could not be there.