> I started a new job towards the end of 2020. It's in a part of software engineering that I love. I'm very lucky to be able to work with incredibly smart people, and improve myself and my skills. But I'm not happy.

Work doesn't contribute that much towards the feeling of happiness. You also need good and honest relations with other people (preferably outside work), play, laughter, being outside, good food, sex and intimacy, sense of belonging etc. Most people who are very focused at work at the exclusion of other things are simply never happy, they just learn to how to deal with being miserable all the time.