1) For some reason it got a lot easier to not care about things I knew, rationally, that I should not care about, right around age 35.
2) The red pill. I don't agree with everything what comes out of that cesspool of a community, but there is a lot of ugly truth in it as well.
3) Kids, job, sleep (and, therefore, happiness). Pick two in your twenties & thirties. However, if you pick the first two, sleep comes later. If you pick the last two, kids probably won't. The happiest people I know picked the outer two.