People say I have a very "smiley" face and comment that they like my smile even when I feel very low indeed.

It's not necessarily that I'm deliberately hiding my feelings though sometimes it is appropriate to do so as long as I'm acknowledging them, but it does help socially if you don't always look as miserable as you feel as people tend to be friendlier towards you.

I don't worry about smiling on the outside if I can be myself when I need to be, unconsciously smiling in public can be advantageous, it sounds as if people who know you well can read your emotions and care about you.
Maybe theres no harmif people who don't know you very well think you may be a cheerful person?