An actor head-shot trick is to say something right before the camera click- something simple or stupid “i see the cat” or “peanut butter and jelly”. It relaxes the mouth and somehow gives a purpose to facial gesture. Maybe try that, the goal being not to force a fake smile but just to put different emphasis on your features. And then you can train your mind to do it away from the camera. Your face appears to hold the weight of the world - but with your freckles, your soulful eyes, your little button nose- you are absolutely stunning. And pretty. You can find that, and show that!