Is it simply the case that your energy is limited, and if you put in effort in one area of your life, you're necessarily sucking it away from another area?

This is a common view, but the research shows it's actually a lot more complicated. Roy Baumeister helped develop the view of

Except ego depletion hasn't stood up as well as a scientific concept after more rigorous replications. Indeed, there's research that shows that the way you think of your own willpower affects its depletion. If you believe ...

First, realize that you actually cannot do everything you should. You can't even do everything you want to do. That's okay, that's just the normal constraint of living in a world where time isn't infinite.

Therefore, it's totally okay to acknowledge that there's a trade-of...

The first kind of fatigue doesn't seem to match up with the energy metaphor much at all. In this case, you think the activity will be exhausting, but when you actually get going, it doesn't feel so bad. The effort is all in the starting, not the doing.

This is the kind of feeling you want ...