Unfortunately, it isn’t always true that the problem is with the person “in the mirror”.
My FIL is now deceased, and while he was alive, was something of a restraining force on my MIL. However, my MIL saw to it that neither she nor my FIL attended my wedding to their son–reason: because when he was a 7yo, he talked back to his mother.
The problems between my husband and his mother go back to his infancy–apparently he ‘broke’ her ideal image of a family by being #6 (she only wanted 5 children), and although his parents went on to have a total of 10 children, she despises none so thoroughly as she despises my husband.
Christmas '97, my FIL decided to gift each of his children with $500 cash from stock market gains. He died in Jan '99, and subsequent to his death, MIL discovered that my husband had been given the same $ amount as his other siblings, and she went bonkers. She insisted that she’d been told that FIL had only given him $300, and demanded that my husband fork over the $200. My husband walked out, and we have not had contact other than happening to be at parties thrown by my siblings-in-law at the same time. My youngest three children do not even know that she is their grandmother, and there is no intention to change that.
Fortunately, my parents are wonderful, so at least my husband got good in-laws. (And for the most part, my siblings-in-law are good people.
You know, at times, I wish there were a way other than cutting her out of our lives, but there isn’t–and it is not the fault of my husband nor of myself.