How do I divide up territories or assignments fairly?

You are looking for an arrangement that makes no one unhappy. That arrangement does not exist. Someone will always believe they got the smaller share, the harder customer, the worse route — and sometimes they will be right, because no division is perfectly even. Your task is not to achieve perfect evenness. It is to choose a rule, apply it without favor, and be able to explain it to any one of them, to their face, without flinching. Much of your difficulty comes from doing this backwards — deciding by instinct who deserves what, then searching for reasons afterward. People sense this. They do not need to win every argument about fairness; they need to see that the same rule would apply to them if circumstances were reversed. That is what you control. Their satisfaction is not. Stop trying to manage how each person feels about their assignment. Manage whether your method is sound and consistently used. The feelings will follow, or they won't — that part was never within your power.