Working Hard, Hardly Working

In my life I have struggled mightily to find situations that allows me to spend my time working at 95-105% of my sustainable long-term capacity.

I have had situations where I worked at 120% of my sustainable long-term capacity, and burnt out.

I have had situations where I worked at 20-50% of my sustainable long-term capacity, and was bored.

Where do I find the situations that lets me work hard but not unreasonably hard? Why is this so hard to find?