Echoing the others here. Up the hay and cut way down on the rice bran. Also agree with Elana regarding her living space, it really needs to be made horse-safe.
If she's not eating hay when you give it to her I would be wondering if her teeth are in need of a check. Young horses' teeth need regular checks to remove any sharp bits that could cause ulceration (and would explain why she's preferring the mushy bran to rough hay). Get your vet to have a look and see if the teeth need floating and if there are any ulcers in there.
ETA: Yikes, just checked your "Good News" thread - she's 13.3hh and 530lb (240kg)?? She really really needs more feed, OP, under those winter woolies she is severely underweight.