More than anyone in the history of Western philosophy, John Stuart Mill is associated with the ideas that humans are individual, rational beings, that our happiness is a matter of individual satisfaction of our needs and desires, and that the sum total of individual human happiness is the highest good.
Mill came to these ideas as a youth of outsized intellectual brilliance.
When J S Mill applied to Cambridge at the age of 15, he'd so mastered law, history, philosophy, economics, science and mathematics that they turned him away because their professors didn't have anything more to teach him.
The young Mill soldiered on with efforts for social reform, but his heart wasn't in it. He'd become a utilitarian machine with a suicidal ghost inside. With his well-tuned calculative abilities, the despairing philosopher put his finger right on the problem:
[I]t occurred to me to put the question directly to myself: 'Suppose that all your objects in life were realised; that all the changes in institutions and opinions which you are looking forward to could be completely effected at this very instant: would this be a great joy and happiness to you?' And an irrepressible self-consciousness distinctly answered: 'No!' At this my heart sank within me: the whole foundation on which my life was constructed fell down.
It's no surprise to modern psychology that getting what we desire is almost completely unrelated to what makes us happy. (Exceptions are people in extreme need or physical discomfort.) And happiness itself is only part of what fulfills and satisfies us in the long term. I would rather be right than happy. Maybe you would rather be connected than happy. Maybe someone else would rather be productive than happy.
So how to live? It's something we might discover, but not something we can figure out rationally. Happiness is a collective function of families, communities and whole cultures at least as much as it is an individual function. Most of us in America who have figured this out are trying to escape a culture of economic production and consumption, to create alternative communities outside the corporation and the housing development. Knowing yourself through introspection and the practice of focused awareness are helpful. It's actually good to know what you want and to go after it with gusto, but your happiness doesn't depend on the success of that endeavor. Give up on security. Learn to be happy when uncomfortable. "Live frugally, on surprise""now I'm listing things that work for me, but it's up to you to learn what works for you.