I think happy and fulfilled are more or less interchangeable. If you ask Google to define happy, it comes back with a definition from Oxford Languages (at least

I think happy and fulfilled are more or less interchangeable. If you ask Google to define happy, it comes back with a definition from Oxford Languages (at least for me it does) and the definitions given are: 1. feeling or showing pleasure or contentment 2. having a sense of confidence in or satisfaction with 3. fortunate and convenient Contentment, satisfaction, convenient... these sound a lot like fulfilled to me.