Oh A, I have been checked out in my own grief, but following along to your story with so much admiration. You have so much inner wisdom and ability to self-reflect, I don't feel worthy of giving you any advice. Other than to say the road from self-reflection to self-actualization is long and lurching and you deserve a lot of self-love in this process, whatever and however that needs to materialize. Hang on to those moments of clarity you have been experiencing and recall them when you are in the actual figurative or literal battle field.
And just to revisit the past a little bit with you... your story about PND and the dynamic that was born with your youngest: please don't spend too much time owning your role in that narrative. Yes, you allowed a certain interplay to develop in your R during that time, but during times of mental illness, we should be able to rely on the 'healthier' of the two halves in a R to help pull us out on to the other side. Instead, your H allowed himself to give birth to the 'true' him when you were at your most vulnerable. He took advantage of your weakness and gave himself permission to do this. And when you came out the other side through the unfolding of your personal growth in your M, he remains stuck. You didn't ask for him to become who he is and you are not responsible for him becoming so. That's on him. We are all flawed, we are all human, but as long as one of us remain 'fixed' and the other lives in 'growth' there is no hope. You can't clap with one hand, as May said. I am writing this while needing to take my own advice on a similar scenario in my own sitch.