But how do you get yourself to do all the cognitively heavy work for your job, day in day out?

Intrinsic motivation. For a career programming, coding should work like a stimulant to a degree. Some people also are motivated by fear/pain. Some by money or status. Most by a mix of traditional motivators.

Well I suppose in my current position there isn't much that is cognitively heavy, it's just...a slog. In previous jobs I had a bit more financial motivation when I worked on difficult things, but I made a change last year to migrate to an easier workload.