It's late where I am and I am typing this on my phone, but when I saw the topic, it reminded me of me. Allow me to explain.
A couple of months ago this year, I graduated my high school and unfortunately for me at the time, my mom wanted pictures. And lots of them. By this time in March of this year, I'd been going to therapy for a while in CBT (I recommend this if you can. My kind therapist gave me different strategies to practice which helped get me through a lot of anxious situations - like the smiling thing - I could never explain this to other people).
Anyways, my mom understood what I was going through, but she didn't REALLY understand. She still wanted senior pictures. Ugh, okay. For me, it literally felt like the muscles in my cheeks felt forced and that it was a fake smile. People had told me that it looked fake which didn't help! I would smile, it would stay for a while and then I couldn't keep it anymore. It'd completely falter within seconds as if I were in pain. You could see what was happening in my mind on my face in other terms. I was always waiting for someone to ask me what's wrong.
It was even worse that I'm a person who shows her teeth when I smile. Genuine smiles look good with that combination. What I had felt like a failing act that not even I could convince myself. I still look at photos from then and feel that slight setback, but then I remember, I've gotten better and it does become better again.
So back to the story, I tried a new strategy. I started to literally reject every bad thought. Anytime I started thinking something negative, I said nope and changed it for something positive. Along with encouraging myself (literally telling myself opposite from what I was thinking), it helped a lot. The first picture I took I was a jittery mess, but guess what? My photographer laughed it off and so did I. In the end, my confidence shot out of nowhere high and I was proud, but moreover just neutral because I realized that it's always been in my head.
So, try watching something funny if it helps. Go out with one friend (maybe not a group right away) where it's not intimate (ex. Dinner) and you can be comfortable. All the while, continue positive thinking like the method I use. Even saying to yourself that you would love to do something even though you may be a little scared might help.
I thought I would never be that photogenic girl I once was, but things have gotten better. I know the feeling, so hopefully I've offered something that will help you.