What is the most damning thing you've said in a serious relationship?

What is the most damning thing youve said in a serious relationship?

In my high school days my top oofs were: "I lied to you about liking you". I put that in a love letter and walked away. I got 10ft before I heard her collapse in tears. Worse yet it was not the first time something similar happened to her by me.

Oh yeah I was a relational nightmare back in those days. But I've definitely brown out of it

What is the most damning thing you've said in a serious relationship?
Post Opinion