Let me start with a contrast.
To me, you are unhappy if you were to firmly wish that you had never been born, or that you had been born someone entirely different.
If instead you are comfortable with having been born, and having been born you, and this feeling flows from good self-knowledge and not evasion, then you have a baseline of happiness. Everything in addition to that, whatever feelings of success or fulfillment, is merely what gives extra degrees of happiness.
Optimal happiness would be the sense that you are "in the groove" in life -- that you are where you judge that you should be, and where you most deeply want to be, doing what you should do, which is simultaneously what you desire to do. For instance, if your dream is to be a painter, and you understand that your circumstances make this a worthwhile activity, then you are optimally happy when you are absorbed in the act of painting something you want to paint.
Happiness has an experiential aspect (a feeling of some sort), but I tend to see it as a process. It is an activity, something that you do, as with the painter. A feeling of happiness does not necessarily have to be felt while engaging in the activity (such as painting). It may be felt afterwards in reflection on your life.
eudaimonia,
Mark