Pajo that is not it. They are telling themselves the things they need to believe to justify themselves.
My w would tell me the things that drove her to the point of BD. I would listen and i think she would also realize it isn't enough end a marriage. so the next thing would be that she doesn't think she is right for me and can't make me happy.
anything that makes them feel ok with what they are doing
I really think its mostly this. Very few things are so bad that if they are brought up as problems and addressed they can't be overcome. From my relationship: When he expressed an opinion, I would explain his opinion to him and try to prove his opinion wrong. He would do the same to me. It annoyed both of us when we tried to prove each other wrong. I would pick on him for wearing these silly jeans. He never took it personally but now, I "gave him endless sh*t for them." They warp the fights to fit what they're feeling. We do the same. Maybe he did take the jeans personally and just never told me. At the end of the day, all he had to do was tell me it bothered him and we could've addressed it and I would've tried to not do it.
But they didn't. They let it get this bad. Yes, we didn't see the signs. Both sides are to blame but I would argue they are slightly moreso because they knew what was broken and didn't try to fix it, we just didn't see the cracks.
M:26 H:32 T:8yrs ILYBNILWY/ "I'm not happy": 2/6/16 D-Day-discovered PA/EA: 2/10/16 Separated: 2/14/16 He's living with OW I moved to different state: 06/16 Currently: Trying to move forward