Look Like I'm Happy

So lately I’ve noticed about myself that when I’m spaced out or just sitting around with people near, I look pissed off. I’m NOT but I look like it! Then people become worried or intimidated by me, like I’m mean or unhappy and they think I’m stuck up and not having a good time. I can’t help my facial expressions, but I really don’t want to look like I’m unhappy all the time, unless I’m posing for a camera then I smile. I try to smile but it doesn’t come naturally. I need to train my face to LOOK like I feel, which hopefully for the most part is HAPPY. Cuz according to everyone I know, when I’m in a group I look pissed off or unhappy! Does anyone else have this problem??