You can choose to be happy at any time. Most people don't understand that, unfortunately. Granted, life can throw a lot of curveballs at us. However, our beliefs guide our mental state, and we are in complete control of our beliefs.
In short, don't LET yourself be happy, CHOOSE to be happy. It's better physically, it's better mentally, it's more productive... there are no downsides.
Happiness comes from a state of calm and feeling well being for your fellow humans. Be in a place where you feel safe and secure. This is essential. Try to do the things you love as often as possible.