Hi WorriedPartner, it seems your post has activated different responses in people, I admire you for hanging in there. "My" survivor has DID or something very near to it so I know how it complicates things considerably.
I'm a little confused by both the guilt/self-blame you express, and the condemnation in this thread. Maybe I missed something, but I gathered that your bf had a gf before you, he left her, then he got with you...ok I am having a hard time feeling shocked....sounds pretty much like every other relationship on earth. People end and begin relationships all the time, sometimes for the "wrong" reasons and sometimes for "right", but people do what they do because it's what they need on some level at the time.
And I take it that you've gathered that survivors running from good relationships is a pretty common theme. Sad, yes. Inevitable, sometimes.
Was it a shame that he left his gf while she was pregnant, well yeah. The whole situation is a tragedy -his being wounded in the first place, which led him to a sad situation like that, perhaps with someone just as dysfunctional as he was, or perhaps he was the one with the issues, hard to say. Everyone loses in the aftermath of child abuse.
But any of us who grew up in a dysfunctional family, know its not always in a kid's best interests for parents to stay together. So I don't jump to the conclusion that he should have stayed with her for the sake of it, or that he made a mistake leaving -or that he should go back to her. It's possible he would have made her life a lot worse if he had stayed. And it's possible that going back to her would mean going backwards in healing.
Anyway, none of that has anything to do with you, IMO, that's his business and his responsibility. He is a wounded person with child & adult parts which makes him very complicated -but he still is responsible for his choices. That has been a hard part for me to wrap my head around. My survivor can be so vulnerable and childlike at times, it scares me because I feel like I'm the one in the drivers seat, and I'm the one dictating the course of the relationship. It's very unsettling to feel like the parent and it's a lot of responsibility.
But it's important to remember he has adult parts too and is capable of being an adult, and IS responsible...we can't take over that for them...it sounds like you grasp this. It sounds like you have helped him empower himself to seek therapy and have maintained good boundaries, which is not easy.
So all this is leading up to my point, and I mean this in the gentlest way possible, because I think you may be shouldering some responsibility that's not yours -it's not up to you to decide whether or not he should have left the 1st woman, or whether he should go back to her. It is his thing to decide.
He may be too confused to know why he did what he did, let alone what to do now, and that's common for survivors to question everything, including (especially) their relationships. I think adult relationships in general can be extremely confusing to the young parts. It's hard not to get sucked down the rabbit hole with them.
What I do think is safe to assume, is that you must have been a safe harbor for him in some way. I seem to recall that he shows you his child parts and you get to interact with them. That is a huge sign of trust (at least it is with my survivor). Also, if his memories surfaced while you were together, you must have provided him with enough safety for those things to come out.
As heartbreaking as it can be to be a survivor's significant other -its also a huge honor, because most survivors do not trust easily.
Remember the saying about a reason, a season, or a lifetime, I think that applies to his past relationship with the other woman, as well as to yours.
We come into each others lives for a reason and bring each other gifts that we take away at the end...whether the end is in a few months, a few years or many years, all relationships have value and lessons to be learned.
Oh, and one more thing I wanted to add, is that I am not sure that a survivor leaving a relationship is always "acting out." I think that's common, and sadly extremely common for the supporters here, but there are a lot of different survivors out there and a lot of different stories. Some men use porn/affairs, others (like my survivor) use rigid celibacy, isolation, or other tools. I think you said it well, not every story is the same (well, you said it better than that.)
Have courage, you're not alone in the fight, and keep your head up because there is a lot of dignity and value in what you're doing.