Unfortunately, because you let them. It is a learning experience. PTSD does not equate bad behaviour. If she was ready for a relationship, she would not be reeling you in and then letting you out again. Perhaps the relationship was comfortable for her, she may not know how to break things off, but there is a certain comfort in having someone around. At any rate it does not make you any lesser of a person because you have decided that you cannot cope with this particular relationship, PTSD or not. I don't think she knows what she wants, but you cannot continue to be her experiment in that area, if you want to survive with your emotions intact. She may not be the evil witch, I doubt she is sitting behind a door with the Bwah-ha-ha on her lips.But everything she is doing points to her unavailability for a committed relationship, and a roadblock in her healing. A committed relationship with anyone right now, is the last thing she needs while she is getting her act together.
You went back once, fine. Even a second time, okay. But you also have to look into yourself and ask yourself why you thought so little of yourself that you had to keep hitting your head against that brick wall, over and over again. Real relationships do not break up on a monthly basis. That's high school stuff, when puppy love abounds. If you are not uplifted most of the time, but hang on only for the minority "good" days, then you are selling yourself short of what it means to be in love, and a potential partner who can give as much as receive.
In all relationships, there will be bad times, and at some point they may outnumber the good for a limited time. But to start your relationship off like that is setting yourself up for heartache. Move on, learn, and eventually you will let your heart open up again, to the right person, for the right reasons.
I