I feel like the idea of flat affect has been so watered down for people. Flat affect doesn't just mean having a "bitchface", and it never has been just that.
How flat affect presents for me:
Limited expressions, I need to be actively thinking about making an expression to have it happen. Additionally, it often takes multiple tries to get the expression I intended. This isn't limited to positive expressions, I also struggle to make expressions conveying things like surprise.
Practicing facial expressions as a child. Being told my smile is "fake" by adults when they wanted a picture.
Having limited tone, I struggle to convey different emotions through my voice unless I am actively attempting. This presents for me mostly as, I will make a joke and people will think I am being dead serious. Multiple times I have made something I thought was very obviously a joke, and then someone will ask me later on if I was being serious.
Shocking teachers and counselors when I do show emotion, but also not showing negative emotions how people without flat affect do. I had a panic attack, and one of my counselors managed to walk right past me crying, because I had a flat expression despite being actively sobbing. The day after this another counselor pulled me to ask if I was ok, she said she was so worried hearing that I'd cried because I "never show much emotion".
I take selfies over and over, because I cannot get the "right expression", which has resulted in me giving up and just taking selfies with my resting face. Of course if I show someone a selfie I took, the conversation always is about how I am not smiling, and how weird that is.
If you have flat affect, please feel free to add your experiences in the comments/reblogs!