Hi HurtAndTired, thanks for being willing to update us with what's going on in your life. You do sound burned out, and your concept of "hitting a plateau" makes sense for where things seem to have settled.
It sounds painful to feel like you've done everything you can, and this is the current result for your marriage. You want your son to experience parents who love each other, who treat each other respectfully, and who spend time together, and that isn't really happening much. It's a loss of what you'd hoped for.
It's good that you're able to share your contribution to the dynamic:
I am emotionally checked out of the relationship. I have a lot of resentment, justifiably so, for all of the mistreatment that I have received over the years, and even though the worst of the abuse has stopped I am still not getting any kindness or emotional support from my wife. It feels like I am putting in 99.9% of the effort in the relationship and she is putting in 0.01%. I don't know how long this is sustainable, or what else I can do to make things better.
She contributes to the dynamic in all the ways you've described (which are significant, serious, and impactful), and you're able to disclose that you're fed up with the negativity, emotionally checked out, and have a lot of resentment.
While I don't have personal experience with a situation like yours, the thought that comes to mind is that it's important you have a place like this, where you can actually be vulnerable, not only about your hurts, but about negative things about yourself, too, if that makes sense -- what I'm getting at is I'm guessing that in addition to feeling unable to share your positive hopes/dreams with your wife, I image that sharing your perception of your flaws with her would not go well.
It's important we have places and people to go to where we can be vulnerable in both directions, about the whole of ourselves -- both our really sensitive yearnings and dreams, and the areas where we fall short and are hurtful... and to know that we won't be judged, dismissed, or mocked.
I want to keep encouraging you to share all of that here -- the "bad" and the "good". Even if we haven't gone through your exact situation, I can say I know what it's like to feel resentful of a spouse. I don't have it all together in my marriage and we do have significant hurdles, and I contribute a lot to those. In that sense, I do understand, because I'm also a limited and hurtful person.
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The paradigm or concept for marriage that you've had or that you've wanted doesn't seem to apply currently to your relationship (support, respect, intimacy, sharing). It would be frustrating and build resentment to keep hoping or expecting for the "marriage" paradigm to apply right now.
It may sound odd, but is there another paradigm or description of your relationship right now that is... reachable? What I'm getting at is -- you're deciding to stay in the relationship, and at the moment it isn't a marriage in the way you would hope for. Maintaining that hope and expectation will leave you perpetually disappointed and may feed the resentment.
If you shift paradigms to: we're business partners in the business of raising S3, or, we're in a group project of maintaining a home for our child, or, we are roommates with some shared tasks and goals, etc, would you say you two are being more successful in terms of one of those paradigms?
Does that question/approach make sense?
If I hold on to the paradigm of "I'm a wealthy person with no money concerns" and I organize my life and expectations around that, I will be perpetually disappointed, upset, and resentful. "Why do I always run out of money, I'm rich -- this shouldn't be happening to me". If I use lenses more in line with the current reality ("whatever it is I wanted, my current situation is that I am a person of modest means and I do have money concerns"), I may have an opportunity to stop feeding resentment. I may one day become a wealthy person with no money concerns, but that isn't right now.
Not sure if any of this will land... and you may have already gone down that road of thought. Just some thoughts to share.
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What is your therapist's perspective on where you're at right now?