Imagine that the laws of the universe changed. Imagine that to maintain the structure you have, you would have to concentrate with all your energy on all the bits that make you, you.
You have to picture the molecules made up of the atoms made up of electrons and protons and neutrons and tiny things that are proven in colliders.
The last thing you think at night is the shape you are and that is held until the next day when you wake up and you think the same so that it holds your shape.
Think of all the effort it takes. The pure exertion of effort that you must give to make your toe a toe and your lung a lung.
If you let go for even a second, you will fade in to billions of tiny parts that never recombine.
It is tiring, right?
That is what it is like to be depressed. That is how much effort it takes.