When I looked at the structure of GTD, I thought that the structure is similar to the advice for people whose room is messy. The messy room has too many things. It is similar to the people who have too many concerns in their brain.
The most popular advice for people with the messy room is “process things from one end.” By the processing, most of the things are thrown away as garbage.
This way of tiding is a good way to visualize the progress with the amount of garbage bags thrown away. By seeing the progress, you keep your motivation.
However, this approach often fails. People cannot throw away everything. Things which they cannot throw away are not organized. Those things gradually increase. The increase brings new confusion about how to organize them.
People with messy room tend to fail to estimate the time to get room tidy. They start tiding up at the weekend, without thinking whether it finishes in 2 days. So it is interrupted due to lack of time. In the case, they do not have time to organize things. The mountain of things is squeezed somewhere unorganized. They do get the feeling of achievement and the motivation to keep tiding up diminished.