There was a $10m prize announced yesterday for creating AI that can solve math at the Olympiad level. We still do not have a cure for most cancers, and there is little profit motive to do so, and this is in part an economics problem. Heck we cannot even guarantee that babies born today will be healthy, and this is partly rooted in economics and social science. So is the problem of mysterious pandemics, potentially bio engineered, and the class war it has ignited.

The world is far from perfect, and the list of hard open problems is huge.

I know this probably isn't what you want to hear, but I do really, really recommend talking to a therapist about depression.

The feelings you describe make a lot of sense given your experiences, but there may be ways to recapture some of the sublimity you pursued, at least in small ways. Maybe medication can help. (If you're so inclined, psychedelics can provide a drastically novel perspective.)