There’s such a difference between happiness and fulfillment.
What is it? It’s a feeling. Happiness, I think, is very in the moment. Someone has made you feel wonderful and you smile all day. You’re spending time with friends and enjoying their company. A good grade on a test validates all your work for a couple of hours.
Fulfillment is something so much larger. It is the feeling when you go to sleep every night, or when someone asks you about your career/family/etc. and you feel completely satisfied. Fulfillment is your entire life. It isn’t a snapshot like happiness is, it isn’t brief. Fulfillment is found with so much more difficulty and so much more work. It can take decades, half a century, even.
Happiness is fleeting. It will go away at the first bad thing that happens. Fulfillment stays with you though, once you’ve found it. It’s more difficult to obtain, but it’s also more reliable. Feeling fulfilled can bring you happiness, but I don’t think the other way around works.
Fulfillment is a mind state. Happiness is temporary.