“Nothing is so fatiguing as the eternal hanging on of an uncompleted task.”

Simplified Meaning:

When you have a job or chore that you keep putting off, it tires you out more than actually doing it. Imagine you need to clean your room, but you tell yourself you'll do it later and keep delaying. Even though you aren't physically cleaning, thinking about it all the time makes you feel worn out. It weighs on your mind and can stress you out because it’s always there, unfinished. Let's say you have an important school project. If you start right away and work a bit every day, you feel more relaxed and can enjoy other activities without worry. But if you keep ignoring it, the thought of it won't leave you, and it keeps bothering you until it's done. The constant worry over something you haven't finished can be more exhausting than actually working on it little by little. By tackling tasks when they come up, you free your mind and energy to enjoy other things fully.