I can share one advice with you all:

Your physique and your thoughts are linked.
For example in you a gloomy, others will see that because of your posture etc.

So, if you want to be positive and feel happy try some of these exercises:
1. Try to force the smile on your face. Yes, just like that, sit and smile, from 30s to 1 minute.

2. Do a "Superman" pose. Just stand like a "Superman" with the hands on your hips for 1 minute against the mirror. It works.