You may be right. But it's important to recognize that, whether it's the PTSD or not, they have a right to pull away. Whether it's the PTSD or not, they have a right to leave.
It might be that your partner leaves you because of their PTSD. If so, that doesn't mean the decision isn't real, or isn't theirs to make. They're responsible for their actions, whatever might be behind their actions, and you can't ignore the fact that they've left just because they have this condition. If they tell you there isn't a relationship anymore, you can't just blame PTSD and go on insisting you're together.
I haven't been here all that long, but I'm afraid I see some situations where people seem to be pretending thier partner has no autonomy because of this illness.
As carers, I think we have to accept that we have control only over ourselves and our own attitudes. We can't keep a one-sided relationship going by force of will. It may be very hard for a PTSD sufferer to be with someone -- that's why it means so much when they are with someone, in whatever way they can be.
But they can also leave. And often they do. We can blame PTSD or something else, but we can't unmake their decision for them, even if we feel in our hearts that it's wrong.