I think you've had great advice, May. And it is so, so, so hard.
I think in a normal, functioning marriage (or even friendship) there's a lot of STFU going on. I do know that my H has always been a bit moany and critical and, at times, self righteous - he's not a perfect man and he doesn't need to be. In good times, I was able to roll my eyes and let him get on with it, and even felt affectionate about his old-before-his-time ways and he was even able to accept a little bit of good natured teasing about it now and again. But that was when there was a foundation of love and trust between us.
When that was gone - and it wasn't just because of his infidelity that it went (the SSM drained the tank for me long, LONG before that) then I had no tolerance for listening to his little moans and groans and would often come back with sarcastic, cutting or argumentative remarks. On paper, I had a perfect right to push back when he was being petty and unreasonable, but all me doing that achieved was to drain the love tank even further. After his EA, it was just not possible for me to listen to his self justifying entitled blame-shifting pathetic nonsense (it makes me mad just THINKING of some of the rubbish he came out with in those times) without responding in a way that made it worse.
I did try the validation, but all that did was give him little rewards for his blaming - little cookies of attention every single time he shoved the blame for his actions onto me. I tried to logic him out of it, and all it caused was arguments that escalated in terrible, damaging ways. I am still a little scared of how he can be when he feels the need to defend himself by lashing out. He has a really nasty side I don't see very often at all any more, but it is there and I really struggle with that. So that didn't work either.
Separation was the best thing, and the second best thing - which I still need to work on - is leaving him in a room on his own to blame and resent rather than provide an audience for it. He doesn't like it. I do think me withdrawing that attention panics him and upsets him. I also think in the short term, it makes him blame me more. But here is where detachment comes in: I really don't care whatever bile is in his head in this moments, so long as he treats me with respect and communicates like an adult the nonsense is his burden to bear and his work to deal with. It is VERY VERY hard but I do think refusing to be around for the pity and blame and 'give me a cookie for not cheating on you today' parties is essential.
It has helped in a way that validating his self-pity and entitlement (which is the DB way, I think) actually made things much worse. It has also helped my development, in that when I noticed that no matter how unpleasant and upsetting it was for me, I really struggled to leave him alone in these moments. I signed up willingly for a lot of his rubbish. And it was because I thought by interacting with him - one way or another - I could get him to process it more quickly, or come to the conclusions I wanted him to, or taking responsibility in a way that felt good to me. It was still control. And that was disrespectful of me towards him, in many ways.