Why is the length of my mental to-do list directly proportional to my inability to get through it?
Because it encroaches on the functional capacity of my brain.
Our cognitive bandwidth is limited, like our current account. Constantly dipping into it reduces its ability to deal with the jobs at hand.

In Psychology, Zeigarnik effect states that people remember uncompleted or interrupted tasks better than completed tasks. This is a human phenomenon that is becoming more apparent in the present times of perpetual clock chasing.

In a recent article in the British Medical Journal, Tom Nolan, a GP says,
“As I run later and later, rather than completing the task straight away, I add it to my list —my brain’s equivalent of opening up a new tab. The later I run, the more frazzled I get, and the more opening up a new tab becomes the answer to people’s problems. Mrs Jones’s headache becomes a neurology referral instead of finding out what’s really going on in her life. Mr Jones’s headache also becomes a neurology referral. With a few more questions and a bit more headspace, I might have realised that the Jones’s have left their gas on.

The more tabs I open, the greater my sense of impending administrative doom. My system runs slower and slower… The longer they’re open, the less important they seem. That’s when it becomes a real problem and the errors and complaints start piling up.”

He feels that if each of his appointments were 15 minutes long, he could do justice to his patients and the paper work, thus reducing errors and complaints.

Ref:

Freeing up some cognitive bandwidth in General Practice: http://blogs.bmj.com/bmj/2017/02/17/tom-nolan-freeing-up-some-cognitive-bandwidth-in-general-practice/

Mental Bandwidth:
http://blog.handy.com/whats-all-this-about-mental-bandwidth/

15 minute appointments:
https://www.theguardian.com/society/2016/aug/28/doctor-appointments-15-minutes-bma-overweight-population