@Sighs, that's really eyeopening, thank you! Coincidentally, my SO and I have reached the exact same impasse. I have laid down the law as to what behaviors need to stop. Don't care how--therapy, groups, whatever--that's up to him. As we're long distance at the moment (regular and prolonged visits usually,) our communication has ground to a near halt. While he's trying to calm down, collect himself, and make his choices, I'm practicing staying firm on my limits. It's been difficult, but there's no other way.
What I haven't formulated as clearly yet is what you mention about "not hoping for a better past." You are right, no amount of rehashing will make what has happened feel better.
Where it gets really complicated, at least in my case, is that most of my issues and boundaries have to do with my SO's penchant for secrecy: From trying to "protect" me from information and hiding correspondences with people on his phone he thinks I would misconstrue to who knows what else. It may very well be that the things he's keeping hidden are "harmless" to me, but I'm a grown woman and would like to make that choice for myself. To me there is a big difference between "privacy" and "secrecy." He's entitled to live his own life and not share each and every detail, but when what's happening to him spills over to me and our relationship, I'd like to be clued in.
While I can't make myself feel better about his secrecy in the past, I'm not sure if I can adequately "let the past be the past" without fundamentally grasping WHAT he's been trying to keep hidden. Unfortunately, it seems like my SO is quick to confuse my needing to know WHAT with needing to know WHY, which is a convo that triggers him for obvious reasons. I don't care how he got this way. I don't need to have a long explanatory convo about all the trauma involved in his behavior. I need to know WHAT I'm dealing with. So "rehashing" seems inevitable to a certain extent. And that may just break us up for good as at this point I doubt he has the trust, faith, and strength to fill me in.
What makes this extra painful is that over the last 2,5 years, we've gone from being great communicators and companions to this...it's a development I find really hard to stomach.