Mi'sen

The only thing that seems to work consistently is to spend a lot of time invoking some particularly horrible thing about the world, but...

... even then, the whole thing becomes rather contrived, turns into more of an exercise in imagination than processing emotion as such.

Sounds a bit like several practices in the Tibetan models - which can easily be very contrived and mental masturbation, they can also be very powerful and/or liberating, when the conditions are right.

I have had massive payoff from meditating on difficult emotions -as they arise-, but I seldom manage to invoce them mid-sitting.

Just making the space every damn day matters. I'm starting to think I need to fine tune the description of that practice though.

You mean in the sense of: opening up to negative emotions, but passing over it in silence if nothing comes up?

But people seem to often have nothing going on. How did they get so happy? Maybe they had nice lives!

It's not happiness; it's not-unhappiness. Low neuroticism. Personality + relative lack of childhood trauma and parental stress.