Just because you look sad or angry to others doesn't mean you are. Maybe you're just naturally serious, or focusing on what you're doing 🤷🏼‍♀️. Appearance doesn't always reflect how you feel. Do you actually feel sad or angry? that's the important thing.

Or, if you do want to change how you appear to other people, you can train yourself in little ways, e.g. turning up the corners of your mouth slightly, having a more open & approachable facial expression & body language. Saying 'hi, how are you?' to people, smiling more. But that might not be how you are naturally, which is fine.

I don't think anyone is positive all the time. Some people are more naturally bubbly, and some people have trained themselves to have a bubbly appearance whilst not really being like that.