Is happiness a moment or a state of mind?

We went to the beach yesterday, for the first and probably only time after getting back from England this summer. We met our friends there, and our four children played together beautifully.

After lunch, the light changed slowly from the harsh, raw energy of midday to the most beautiful, gentle, silvery haze, with the line of the horizon almost invisible but for the metallic glint of reflected sunlight, and the sea swelling lazily and occasionally producing fierce flashes of the dying light. We took turns watching the children and sitting on the sand, and as my little daughters played happily and I took a last dip in the sea, it was so easy to stay in the moment, to feel my own happiness, to remember why we live here, so far from our families.

 I've been questioning that decision recently, as I do all my decisions, endlessly, and the carousel of thoughts always stops at the same place; practically, there is nothing to go back for, and emotionally, whatever problems we have don't have geographical solutions. I think happiness probably is a momentary thing, but contentment is a state of mind, and probably reached far more easily by conscious control of one's thoughts than it is by changing the externalities of one's life.