Once you start 'succeeding' at what you're doing it tends to feel like it takes less energy, and if you start to feel good about it, you'll also tend to feel more energetic, so it forms a self reinforcing loop.

It won't work like that for everyone. You have to actually be enthused about the idea of succeeding at whatever it is you're doing and not push so hard you burn out regardless of success, but if you can get into that frame of mind, things becone easier.

Spoon Theory describes it pretty well IMO.

Some of us only have a handful, others seem to either have enough to serve soup for an army battalion, have many of others' available to them, or both.