I like Aristotle's definition of happiness as a state that happens when you act "in accordance with your nature." I know it sounds vague but in practice, it works. I love to teach,. When I teach, usually it "makes me happy" to do so.

So, rather than trying to attain and hold a state of happiness, I think it's better to see it as a side-effect of living life with a sense of alignment and purpose. Oh, I get it that there are lots of ways to critique this stance (is Dexter the serial killer 'happy" when he is killing his victims?"...but for the general case, I think it holds up well enough.