Having read your other post, it's easy to apply what
@EveHarrington is saying
tell her that you are not going to be present when she is exhibiting behaviours that put you down, and then if she continues on this path to ruining the relationship that you won't stay in the relationship.
When your gf starts talking crap to you, you tell her to stop, if she doesn't, then you walk away
She picks at something you do, again, tell her to quit, if she doesn't, walk away
that means legit leaving the situation entirely. you have a right to a relationship where you feel like an equal. Shitting on someone is not excusable because of PTSD, though we all tend to lash out when we are symptomatic. But we have a right to not have to stand around and listen when this is happening.
You can encourage her to go back to therapy, but are you prepared for this to keep going on and on if she doesn't? What sort of boundary can you make, that is not an ultimatum, to help her get the help she needs?