The single quality that makes the most difference to whether you are happy or not is thankfulness. If you are thankful for your life, and can see good and hope in whatever life brings you, you will be happy.
It is possible to be thankful in the midst of the most dire situations. People who are depressed have not usually had more suffering, more failure, more disappointment. They just spend more time thinking about the hurts and disappointments.
People who are happy are not happy because everything goes their way but because when things go wrong, even very badly, they can still find reasons to be thankful.
Garvan Byrne is a perfect example: