I understand that you believe yourself to be a strong person, but it doesn't help anyone to use this as a reason to stick by someone. That is, PTSD is a whole 'nother can o-worms, and until you've actually gone through the supporting role, you never really know if you can do it for sure. There are TONS of people out there who are "strong" yet wouldn't be the best supporter for someone with PTSD.
You seem like you want to be the white knight....Yes, a very real phenomenon! (I dumped a guy like this....he made everything about me being "sick" and trying to fix me.) The truth is that your role is ONLY to support. She MUST be the one who seeks out treatment, she MUST be the one who works hard on her healing. MANY people need to hit rock bottom before this happens. (In my case it was nearly losing everything, including my life.)
Why are you still paying for kicking her so long ago? This is not a healthy relationship dynamic in the least! Partner does wrong, partner repents, couple moves forward. This woman sounds like my mother in that she holds EVERYTHING against you! OMG I have things from my childhood held against me, from 25 years ago! (I was a child.....SMH.)
She feels deserted? UHM, who's fault is that? Why can't you see her role in being deserted so many times? You're feeding into victim mentality by pitying her for being deserted, with the attitude of "I'll stay by you even though everyone else was SOOO mean and left you!" Everyone has standards for behavior. Seems to me like she thinks she can behave whatever way she wants and then when things go south, blame the other person. Yes, she has a mental illness, but at the same time, she needs to take responsibility for her role in all of this.
And I know that sometimes the courts DO get it wrong, but I still wonder if she had a major role in losing custody of her kid in that you are in America, the country that loves to give the mom custody. Maybe you're buying into all of her manipulation crap and she is simply bad mouthing the ex? Maybe that kid IS better off with his father? (And if not, God help that child as he is going to need it.)