I call my mask a 'social script', like computer programming or a screenplay.
In almost all social situations, I prefer to observe. I prefer to listen, and only respond when spoken to. I enjoy people's presence without the need to interact with them. But that's not how people usually socialize when they meet up with friends / coworkers / etc.
For my script, there are certain regular experiences that happen which I have 'pre-recorded' responses for in my brain. My responses are quite automatic now, but I wouldn't say they are 'me'. Small talk comes easier to me now, but I hate it, so on a low energy day I won't mask up to do something I hate. My preference would be to be silent.
For my programming (more for behaviour I guess), it is again a sort of script. Much like my preference to be silent, I need some sort of input, environmental stimulant, to then garner a response from me. It doesn't come easy to me because things like smiling, I only naturally do when I actually feel joy inside me, and I am a fairly low emotion person. I find it pointless to smile when you don't have that feeling. My preference, or natural state, is to keep my face the way it is, unless I am communicating an internal emotion. Facial expressions and vocal tone for social situations are conscious efforts on my part and I spend a lot of energy to pick the right ones, according to my script.
Over time I have been judged and criticized daily for these small 'weird' mannerisms to the point that I have 'fixed' them to avoid making a situation uncomfortable for other people, so now I just suffer in silence. Trying to undo all that now.
I think if your behaviour is a burden to maintain, and causes burnout over time, then it qualifies as a mask. But to nitpick and figure out if every little thing you do is mask or you, is a waste of time. There is a gray area to this, between mask and you, that you don't need to question (as others have said, since everyone, ND or not, does this to some degree). But if you want to figure it out, I think the most obvious / burdensome ones are really the only ones to concern yourself with.