Some great advice on this thread.
I do think our contemporary society has been fed something of a lie about happiness. We are continually told we'll find happiness when we get that next thing, next experience, next relationship even - that we must be continually striving for more. The more we strive, though, the more that nagging discontent takes hold and we find ourselves left wanting more to fill the gap created by the lack we are feeling. Ads on TV and social media really do perpetuate this, and make us feel like we are not enough, that our lives are not enough, that we should be getting more and doing more and being more.
Social media also brings the burden of comparison which can lead to discontent because we look at other lives, that often look so perfect on their Insta feeds and FB stories, and we find ourselves wanting. Yet the reality is that those snapshots in all likelihood do not give a wider picture of that person's life, and they are probably experiencing all the normal ups and downs as much as anyone else.
I think that this last couple of generations has definitely taken hold of a sense of entitlement about happiness - that somehow we deserve it, can earn it, can get it and if we don't then something's wrong. But like pp have said, I'm not sure we're built for 24/7 'happiness' - if we lived in that level of perfection, we'd probably get very bored because the human psyche knows that the highs come because of the lows.
I think contentment and happiness are different. Happiness comes in snatches and we should grab hold; the practice of mindfulness is good for this, and gratitude. But contentment is more of a steady, underlying mindset and I'm not sure can be achieved by things, or even relationships, because there are always imperfections and issues. Having children has brought me a lot of happiness but I wouldn't say that they have fulfilled me in the sense that I couldn't possibly know fulfillment without them - that would be putting an impossible burden on them to be the things that were there for my happiness, rather than being people in their own right with their own paths to find.
I live with long term ill health and am often sick and housebound, so many of the the answers mentioned above are not available to me (volunteering, classes etc) - but it's very much in a mindset of gratitude for what I do have that I find contentment. Like a few others here for me my Christian faith is what brings me the most contentment in my life, filling a void nothing else can and releasing so much joy in me as well. And for me it's finding my passion in life, which is writing books and articles, that has led me deeper into contentment as well.
I think you need to locate where your passions might like. You say you don't care much about any causes - maybe you haven't yet found the one you might be able to invest in. Is there anything you feel more affinity with in the charity sector? Animals, children, elderly, disabled, homeless? Even a small spark of interest could grow into something huge if you allow it to by investing in with your time and energy. Sometimes it's a case of making a gritty decision; however you feel, you are going to do this because you know it will do someone else good, even if it won't help you. And, in the end, you will find it does help you - perhaps nothing like you expect it might.
All the best, OP. My best advice is to look to the good of others, and look into matters of faith.