Sorry op but he just sounds worse with every post.
Why stay with someone who has held a grudge for fifteen years? And who you even remotely think is the sort of person (sounds like he absolutely is btw) who would use it to treat you shitily. If you hold yourself accountable for things in life and seek therapy for growth, why do you not hold him to the same standard?
Why, rather than him realising he was mad at you and seeking out therapy for himself did he decide it was ok to be horrible for 15 years instead? And you can't say he does it without being conscious of it. Because that's bs
And we all know it. He does it because he needs any excuse to treat you like shit. Because he WANTS to. And you know this!
What you need to do is stop sweeping that knowing under the carpet. Abusers train us to always make excuses for them. And then we go on that way for every other...sorry but, shitbag, who gets close to us. They use our mistakes against us to make us feel guilty, to tell therapists we are the bad guy, to excuse their continued rotten behaviour. And they prey upon the fact that because we are good and love people close to us, we assume other people are the same. He is not the same as you. He doesn't want good and joy and health and happiness for you. He wants you sad, stressed and exhausted. He wants you punished. That's not a partner, it's a jailor.
You have to remember (some) couples therapist may want to keep you together longer as it PAYS them. That doesn't necessarily mean it's good for you.
I'd consult your individual therapist and talk with them about the situation. I'm sure they would focus on building up your confidence so that you no longer accept shaudy treatment and find it easier to see it for what it is.
You don't need to spend your life tied to someone who doesn't like you. Certainly not out of guilt for some old mistake. Or to benefit kids if they are involved (because it doesn't do that, it just shows them relationships should look crap). You can absolutely leave, survive and thrive.
And if you are afraid to because he will make it difficult, remember that the comparison could be another, what, 30 years of things the way they are now. Only worse because..they always get worse.