I was yelled at all the time as a kid, my mother did a lot of belittling and that sort. Generally when I did anything wrong, it resulted in an hour of two of being yelled at and belittled in which I yelled back for as long as I could and hten broke down and cried while my mother continued to yell. Beyond straight out yelling, sarcasm and snide remarks were employed. I vowed not to be like that, but I struggle with this constantly. I don't usually hang around this forum much because I don't feel the folks here understand that its not just something I can just 'not do' like I choose the color of my shirt in the morning.
Like OP, most folks who don't have a tendency to yell don't understand what those who struggle not to have to go through - when I get angry and frustrated, that is the only thing I want to do. When my daughter continues to push my buttons (generally by hitting her little brother) I just come unglued. I try to vent it into statements of feeling like 'I am angry!', or just make angry noises, etc and not attach any judgement to them, but I know this still affects my DD. I see her yell at her little brother in turn and I know its because I yell at her. I hate it, but its not as simple as 'not doing it', its so part of who I am and I'm very frustrated with that.
I feel like I just don't fit in as a GD parent because I struggle with this. I really wanted to abandon this post even after I took the time to type it, but I'm fighting that desire and going to post anyway because I feel there are others out there that struggle with this and I hate to have them feel alone here. I hate that there is the idea that unless you can conform to the principles of GD, then you aren't "GD" (or AP or NFL or whatever). Some of us have a harder journey getting there and need support along the way, not to be judged because we are imperfect. I believe GD is the right thing to do, its my goal to get there, but I'm not there yet.
Maybe we need a 'Adult Children of Yellers Anonymous' tribe or something