He had PTSD when we started dating. It's a huge learning curve. Nobody knows what they're doing when they start a relationship with a sufferer. I messed up a lot. I still mess up.
I lucked out. He was being treated when we started seeing each other. He had even done inpatient treatment. He was open about his PTSD from the beginning and I started researching right away. Now with the way the VA is backed up he hasn't been able to continue getting the treatment he needs, so he is highly symptomatic and not doing well. Feast or famine.
If I were in your shoes, I'd work on some "me" stuff.
First, be better about self care. We all need that. Make sure you get enough sleep, good food, and exercise. Find something that relaxes you and make sure you get some "zen" time. It sounds trite, but it's emotionally exhausting being a supporter and getting no support for yourself.
Secondly, have a think about your limits. What can you realistically tolerate in the long term? She may never improve... what can YOU do in that situation. Work on boundaries. They save your sanity.
Did you say you're researching PTSD? I find that also helps. That and talking to other supporters here. It takes a heap off my mind to talk to people who "get it".