That's why I started "fakin' it til I make it."
Like, I'm a terrible hallway person. I hate making eye contact with people in the hallways at work. At my most sensitive, it's obvious by my face I'm deliberately trying to ignore them. So I know where you're coming from.
I'm not agreeing with your mom, and you sound like you know whatever your face says is a reflection of how you feel inside. Some people wear their emotions on their sleeve, and it sucks because everyone tends to know exactly how you feel just by looking at you. And it sucks because when you're in public, it feels like you can't have a private thought to yourself because someone will see it on you.
So, like I said, I started going cool and confident, regardless of how I feel inside. Stonefaced, if need be. And I really do think the simple act of trying to hold myself a certain way has caused my mind to follow my body, instead of the other way around. It starts feeling kinda natural and then....yeah, suddenly you're kinda feeling that way.
Also I used to ruminate a lot around family. Just pursue unhappy lines of thought in public, and it'd all come out in my face and my attitude. I really started trying to push off "thinkin' about stuff" until I'm alone. Depending on how much time you have to spend around your mom, maybe that's part of the problem. My mom's a "watcher" too, and it bugs the shit out of me. People in my family seem to get some entertainment watching my thoughts play across my face and it gets pretty irritating, to the point i'll see her watching me for like 10 or 15 seconds and finally snap and go "WHAT?!"