Hi nodoover,
my pwBPD is ruminating - that is a new one (but I suspect a quite common one). Sounds like you have a grasp off his emotional state

BPD husband brought up yet again a recurring major issue with him, that he feels the need to tell his daughter the truth about his marriage with her mom, mainly he wants to spill the dirt he held in for her whole life now that she is adult. (26)
He tried to a few times this past year and she stopped him. She told me she is setting boundaries with him. Doesn't want to deal with his negativity in her new marriage.
So tonight he brings up few days before Xmas (no surprise here) that his life is ruined because he can never tell her the truth.
I try to listen yet again (for the zillionth time) and say to him what if you can never tell her what you want?  :)o you think you can find a way to live your life without it? Answer NO
That is not validation, this is problem solving. I know, it is SO hard not to do it. We codependents are all soo helpful and it is working against us... .
I once again try to talk into counseling to which he says I tried that didn't work.
I don't know what to say to him. I empathize that I know he feels hurt but that he needs to find a way to live beyond her.
Your H is obviously stuck. He has a problem (wanting to communicate something) and an unreachable solution. This of course is a situation which is distressing keeping him from thinking clearly and selecting another workable approach to deal with the core problem - feeling not so well about his past and the relationship with his D. The past/relationship problems are projected into the "get it off the chest" solution. The fact the the solution is unreachable is frustrating and the feeling of frustration is dysfunctionally comforting the pain at his core.
Tough situation. What options may there be to ponder?
We have this workshop
TOOLS: Dealing with ruminations. You may want to take a look at it whether anything useful is in there. It is however more aimed at people trying to deal with ruminations while your H sounds more fixed on a particular solution that is out of reach. Another relevant workshop may be:
Are you holding on to fantasies about someone who has hurt you? which may be helpful to get some feel for rumination underlying thinking. Last but not least it may help to take a look at the
LESSONS OF THE LEAVING BOARD which outlines the detachment process from a past relationship - a process which possibly has not been fully concluded for your H yet.
I found this book quite useful to deal with my own ruminations.
The Post-Traumatic Stress Disorder Sourcebook - Glenn R. Schiraldi, Ph.D. If he is a reading type he may find some approaches that will help him in the self help part of the book. And if he is ever considering therapy while it does not cover BPD therapy it does cover a range of choices which helps to formulate some opinion. PTSD has some overlap with BPD in terms of symptoms and underlying mechanisms, is less stigmatized and he may relate to it as he is struggling with dealing with past trauma.
He could also try to write things down. That way he communicates it to paper but she does not have to read it. Or does not have to read it now, maybe she has some interest to read it in the future. Both sides get their choice.
I didn't have a clue when we retired here 8 yrs ago he would spend his days ruminating over and over about her. I didn't know what he had in the city and thought it was his job he was angry at so I encouraged him to retire. I thought he would be happy but instead he has regressed to wanting to die. (No plans ever so I am thinking it's mostly for attention)
Part of me thinks I am getting codependent and overly mothering him and part of me wants to know what to do? I am starting to resent my life with him, hard to find joy living with such a Debbie downer all the time.
It is clearly bothering you so thinking about how you limit the support for his quest may be in order. It is an unsolvable problem and of course a little attention to it is needed. But it is also his quest (bottomless hole) and you can't problem solve or validate his way out of it.
When validating it may help to focus more on the core of the problem - him feeling depressed, relationship to D, feeling old, past pain bubbling up - to raise awareness rather than his solution (indiscriminate dump). Establishing smaller, intermediate and reachable targets for e.g. relationship interactions with D may be also an option.
Faced with intractable problems we need to take a step back and think out of the box. Your current approach is tiring you out . Suggestions above may or may not help but all require energy. What is critical is proper self care and boundaries/limits. Even if you can support a change it will take time. So for these festive days maybe do something good for yourself first