Comments made about my facial expressions

All my life family members and strangers have commented on my normal facial expression, it's been described as serious, angry, lost (I really don't get that), a frown and not cheerful. The sad thing is only up until last year when I realised I had social anxiety did I notice that people where right after seeing myself in mirrors and window reflections just that the whole time I was in denial and thought I looked content not all the things they said. Now I'm trying to make an effort not to look so serious or unhappy but it's not natural for me to walk around with a grin on my face for no reason, I've tried practicing in the mirror which worked for a while when friends noticed I was smiling a lot but then I always seem to go back to my normal expression. Has any one else been told they need to smile more? Or had comments made about their normal facial expression especially in public?