This is a post I would strongly mention as not being completely researched.
Being useful is a major ingredient of Happiness . But that is not all. Happiness is much more than just being useful. Being wanted is another requirement.
When you just are in the company of people you love and do nothing just hold hand and look at each other you feel Eternal bliss.
Between being useful and Happy the former is a subset of the later.
To be happy you need to be physically healthy, mentally peaceful with positive thoughts, emotionally having healthy relations with people.Every day you look forward to creating pleasant memories and being productive. You contribute more to nature than what you consume. I have some writings explaining these but am still learning.
I am sure you will find a lot of stuff to support this thought of being useful but have objectives of being more than just useful.
Look forward to more enlightening articles from you,
All the best,