Surprisingly, I've found the most fulfillment from raising and caring for my three-year-old daughter. I always feel immense pride when I'm tidying up the house and come abroad a pile of drawings or an arrangement not only of her toys, but whatever things she's found of mine (ie; the Peewee Herman action figure I keep beside my PC). But as fulfilling as it is, children are also inadvertently vampires and drain you of nearly everything you have. So I have to take pleasure in other things as well, like reading fiction, playing games with my wife, and spending time in nature. What I really need is a good mushroom trip through the forest, but it's been several years since I've been comfortable enough to step away from responsibility and parenthood to do something like that, even though it's a wonderful tool to bring one's self back to earth and get your brain firing again.

Highly recommend trying that if you're in the same mental cycle as OP. Definitely don't opt for children if your goal is fulfillment, though. You may gain something huge, but you're also signing off on the other side of your life. It's certainly not a good fit for everyone. You'd think this obvious advice, but a lot of people do just that and wind up miserable and resentful, and their kids in turn grow up with less love and respect than they could otherwise have.