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This is a good question, and you're smart to attend to this problem as it will make a big difference in how you're perceived by people. Smiling and facial presentation can be learned as a skill and practiced so that you do it more often whether you have the emotion behind it or not.
Start with making a decision that every time you think of it, you will smile. Alone or in company...move the outer part of your lips up gently and raise your eyebrows to open your eyes wider. This is a very attentive face. Look up and out and smile. Put something around your wrist, and every time you see it, do the smile. This brings the cue to smile from the outside instead of waiting till 'you feel happy' to smile. Pretty soon, you'll remember very often to open your face up so that others see you as an inviting person. People will naturally reflect the smile back to you, which is great feedback and encouragement.
You can read more about the importance of this by googling 'smile cues' and facial impressions'.
My best to you.
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