What counts as sustained? Does a bad day, week, month, or year disrupt it? Or is a year or decade of general happiness mean it is sustained? Does it have to last until your death?

Maybe it requires the context of your first sentence about seeking happiness.

For myself, I would not say I seek it, but that I chose it. I feel I've been generally happy nearly always. Sure, there have been stresses, losses, tragedies, tears, and such. There have been extended periods of feelings of inadequacy and of being trapped. Yet, even through all such events and periods, I'd say I tend to be happy. And the last major part of a decade has been the most fulfilling and blessed part of my life yet. Sure, it could come crashing down. Or it could get even better! I chose to be happy with where I have been, where I am at, and look forward to where I'll go.