I can believably (I think) smile on demand. It took some research though, and several months of practicing in front of the mirror. At first it was about smiling not only with my mouth but with my eyes, so I had to learn how to move my cheeks slightly up, and independently from other muscles in the face, and then make that movement whenever I had to smile. It also helps a lot to show the teeth instead of just curving your mouth.
Now my smile has dimples if I want it that way, and I look o.k. in photos, so I believe I got that out of the way. I would advise to do the same, practice moving your cheeks in front of the mirror, and learn the differences between a real and a forced smile and work on that.
For starters, try just closing your eyes slightly. It will make a difference.
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To build may have to be the slow and laborious task of years. To destroy can be the thoughtless act of a single day. - Winston Churchill