My feeling is there is neither true path to happiness nor a definite way to sustain it. Any positive change in time will turn to be its undoing.
He argues that true happiness comes from being-at-work in accordance with virtue. And that virtue can be cultivated through habit. So in his scheme, you can work towards sustained happiness.
Three of the important virtues he devotes separate chapters to are: courage, temperance and practical judgment ('phronesis', sometimes translated as 'prudence').
Of course, everyone develops their own map on how to understand and go about their lives. Just thought I'd share a pointer to the one in this work.
Its very easy to say these things so I never put much weight to them. I am just going off my own experiences.
What you might consider virtuous like helping others may lead down to lonliness and feelings of uselesness when circumstances change and theres no one around. If you work on being independent you may one day find you have a longing to connect. When being carefree seems to make you happy, one day it may lead to making terrible decisions.
The human brain, and all of life seems to be built on opposites, I don't believe eternal happiness can exist, at least not for everyone.
Sounds very much like the definition of Dharma.