FWIW I don't feel that the writing failed. And I don't think she went mad, or something. She just buckled, and that's as close a depiction of (powerful or not) people buckling under the stress and loneliness and emotions that I've seen in real life or other imaginary worlds of literature and theatre, as you can get. IMHO, YMMV.
Disclaimer: I've read all the books way back when they weren't even wildly popular, and consequently didn't pay much attention to the TV show until it started outpacing the existing books. Nevertheless, the only gripes I am experiencing with the show mostly relate to mindbogglingly bad tactical decisions related to warfare. The characters, with whichever minor imperfections, behave exquisitely in character, to my tastes.