I'm younger than you - probably median HN age - but I've felt the exact same way for 10 or so years now. Just like you're saying, my generation has it objectively very good from a consumerist abundance PoV. Part of it is lower quality materials and manufacturing, but even if you disregard that factor, it almost certainly still holds. But that increase in material wealth hasn't brought happiness; if anything, it has done the opposite.

The optimism decline feels like it really started around 2001, and the downward curve has steadily continued ever since, down and down. Worst of all, it seems completely warranted, even if you try to look at it as objectively as possible, disabling news-headline-induced emotions as we're all prone to. The world has gotten too complex, everything too intertwined, making for an increasingly volatile and dangerous system.

And besides that major trend, it's also just many of the smaller things getting worse by the year - enshittification of both physical and digital goods, declining government services and so on.