When I read about Jean-Paul Sartre's philosophy portrayed in his novel La Nausée (Nausea) by the character Antoine Roquentin, I suddenly realised I had overlooked two aspects in my earlier model about achieving happiness.

At a first glance 30 year old Roquentin appears to be free : he has a private income, no family, no job, no friends, no ties at all, and can live where he pleases.
Sartre however argues that Roquentin is not really free, but instead merely non-attached or uncommitted.
He is, Sartre explains, a mockery of freedom and is running away from it. Roguentin is decidedly not happy.

The first aspect I overlooked (or rather did not consider) is that one happiness factor by itself is not enough. Only the combination of a number of factors can achieve happiness.   Roquentin has his freedom, but he has no friends, no-one he can really talk to, no thoughts other than casual ones. This makes his freedom by itself empty and useless.

The second aspect I missed was also overlooked by Epicurus himself. It is an additional and important factor for happiness : a sense of purpose.

Therefore here is my modified list of factors for achieving happiness :

Perhaps it is possible to be relatively happy without a sense of purpose for a short period of time, but soon (I believe) one gets tired and bored with this. For me personally a sense of purpose (like for Epicurus and his friends) is achieved through occupying my mind with appropriate subject material to contemplate, analyse and pass on to others.

These can be ideas I have about music, bridge, philosophy, or any other area which catches my interest, and may be passed on verbally face to face, or through books, or (like these days) through my lessons and Blog entries on the Internet.
For others their sense of purpose (after retirement) may be achieved through physical activity, such as volunteering for charity or community projects, getting involved in gardening, selling produce on a local market, etc. etc.

A sense of purpose is particularly important because it tends to utilise and enhance the factors of Freedom, Thought and Friendship.

But watch out ! This Sense of Purpose factor is also rather fragile and can easily become your "Achilles' heel".
For if your efforts are not being appreciated by those you had meant to benefit from it, then your happiness and sense of fulfillment may disappear and instead turn into disappointment, frustration, even depression.

This happened for example to the renown physicist Ludwig Boltzmann, who's ground breaking work on entropy was not recognised and confirmed until weeks after he had in despair committed suicide in Vienna in 1906.