Sorry you have been going through this difficult situation for so long op. I am not quite sure I understand what you are hoping for in this scenario though?
As you have experienced, people with serious mh issues, often don’t have sufficient bandwidth in their head to be unselfish and look at things from another person’s perspective and therefore often don’t make great spouses or parents through no fault of their own, because just getting through every day is a struggle. Will your dh really change after such a long time?
Another factor is that good mh often depends on having a flexible attitude to life and rolling with the punches, accepting reality, and adapting to change. Therefore people who have a very fixed idea of how parenthood might be, cannot cope when the reality is different.
Also our brains are receptive to the things we think and say out loud, over and over! After a while we start to believe what we tell ourselves repeatedly.
So it sounds like your dh is pretty “stuck” mentally speaking and you don’t want to follow down that path yourself.
** Edited & corrected: sorry just saw that your son is a young teen and not twenty,
… you are divorcing your dh, you have endured many years of trying to be patient and understanding. Now you are splitting, I don’t really know what you can do to change your dh’s mindset after all of this time, or even if you should be trying op? Or whether you can put any firm boundaries in place that will stick?
Your son is nearly grown up! There won’t he huge amounts of co-parenting involved will there? Why is it necessary for you to do anything further at all? Is your son too young to take the lead on logistical arrangements? Could it be that you have been holding your family together for so long that you can’t let go of that role and that you are a bit “stuck” too?
On the other hand your dh could do lots of work on himself! It’s probably a bit too late to be doing this but he needs to work through his parenting issues with a therapist and not lay it on you, or God forbid, on his son.
Tbh if his narrative is that he is blaming the stress of parenting on him becoming mentally ill, or he is saying that he could have been more successful in life if he hadn’t had the responsibility of a child, and being a parent has held him back professionally, and that is making him depressed, you would be well within your rights to challenge these narratives and tell him he needs to go away and sort himself out in order to have any hope of co-parenting with you in a civil manner.
And until he makes progress, you have every right to distance yourself and just focus on your own life and your son’s and try and achieve some peace and contentment for yourself 💐