Trapped in Pretty Lies

The words that Harry Potter writes into his hand with Umbridge's blood quill come true. Harry Potter can't tell any lies. Meanwhile, Dolores Umbridge can't remember a big chunk of her past. Why is she having nightmares? And why does she feel guilty every time she sees her paintings of cats? Can Dolores be redeemed? Or will she die evil with her past covered in careful pretty lies?