And I don't even care very much about solving this problem, either--I've just become more laid-back in general.

Maybe that's a good, adaptive thing for you! Although maybe not if you haven't figured out reliable methods for actually cheering yourself up. (Just getting work done is a very reliable mood-booster for me, when I'm up to it...the second best is usually proxy work, like doing dishes, cooking, baking, exercising, which feels productive and gives me the dopamine hit but is usually lower-threshold to start doing.)

Have you tried happiness-tracking software like forget.io? I think this is an awesome strategy to learn what actually works to cheer you up. (I was doing it, but the company has a US phone number and so as a Canadian, I was paying exorbitant text-message rates.)