I'm sorry your family went through what you did. I find it grounding when I think of what others have gone through. I've learned you can't form accurate judgements because all of us experience challenges that we don't always allow people to see.
As a parent, I understand self sacrifices for a child. As a spouse it seems to be different. You stop being commended for the self sacrifices you make for a struggling spouse. There's pity or a push to move on. I am an independent person. With that, I don't have much of a support group. I think it's part of the reason I stay even when I know the relationship is less than ok. There is also the wishful thinking. The hope that people can choose better paths and make changes and then there's the uncertainty that the grass is really greener on the other side. My fears are that I would at least not start out with full custody but more likely a 50/50 split because his parents have money for an attorney. There would be a loss of control on what his behavior is like around our child. How different would it be from dealing with an unstable spouse inside marriage to dealing with him as a coparent outside of marriage. For me it might be better, for my son I am not sure. I know none of this is healthy, these are the thoughts that go on in my head.