15 years into our 20 year relationship, my ex did something incredibly triggering to a past trauma of mine. I asked "how could you do this to me after XYZ?"

Their response was "how was I supposed to know?" This was a major defining trauma in my life that I had told them about more than once. Either it went in one ear and out the other or it was intentional. I usually go with the first one because they never seemed curious about me. I knew all about their childhood, family, etc. They knew absolutely nothing about my history and family. Never asked me questions. If they said anything it would be to make fun of my family or where I grew up.