I think the key to finding happiness is becoming a happy person. Consider Anne Frank, who was able to find joy in life while hiding from the Nazis. Consider rich people who hate their lives. You can be joyful or sorrowful no matter what your circumstances are, so it's not about finding the right relationship or the right house or the right job. Happiness isn't a goal that you reach when you finally hit the jackpot, because any shiny new thing will lose its sheen in time. Happiness has to be a quality you carry with you and a way you live your life.
So, how to become a happy person? Well, I could give you lots of self-help type advice about journaling, yoga, etcetc, but I'm not really an expert on any of that stuff and I'm sure you could find out more about it from others.
I think that knowing yourself is of great importance to this whole question. Once you know enough about yourself to know what makes you sing and what doesn't, you can figure out how to build fulfillment into your life. Working on challenging and fulfilling things will absorb you and grow you and show you where your joy lies. You'll find yourself shedding the things that don't enhance your life because you simply need to make room for the positive stuff you've brought in. Example, if you take up woodworking as a hobby and fall in love with it you may find you no longer have time for soap operas, even if you once thought watching soaps made you happy... if you're actively working towards enriching yourself the best activities will float to the top and the others will sink.
Ultimately, some people just have a baseline. They're naturally optimistic or naturally cantankerous and might not ever change. But a naturally optimistic person can live a surprisingly unhappy life even though they have a more "happy" outlook if they don't assess their life realistically and find ways to be challenged and productive. And a naturally cantankerous person can have a surprisingly fulfilling and satisfying life if they work on things they love, even if they may not seem like they're "happy" most of the time to the casual observer.
That's my take on it.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.