I'm not sure, perhaps a poor candidate to respond as I realize I have my own emotional baggage and fears. But as a sufferer, and also a sister to my own sister whom I don't know if she has ptsd or not (it is possible) I feel I can only give love without expectation, and within my own limitations (and hers as to what she can accept). One could say I have to because she is related, but no, I had to sever ties many many years ago with 2 other sisters because it was too damaging.
I realize that romantic relationships are a different issue, though I think of commitment and love as 2 different factors; one can easily have either one without the other (eg commitment and provision in a loveless union; love but no fidelity or such etc, though I realize that's a broad example) . But as a sufferer, no matter what the relationship, it's terribly depressing to see disappointment in the eyes of the other person, when you can't 'do better'. I suspect if one is a 'walking train wreck' it is best to not be in a relationship, though it's hard to not feel (always) like a walking train wreck for me! It sort of adds to the cycle of depression and discouragement to feel that one is not entitled or capable of that as well, and are causing 'harm' or 'damage' to another As a consequence relationships (all) suffer, perhaps self-fulfillingly? We can't know if we don't try but we can't try for fear of the worst.
Not sure if that's helpful. :(